GIFT  OF 

Pacific  Coast 

•Tmirmal      of*    TJiiT*st*  no* 


OUTLINES  OF  PHYSIOLOGY 


JONES  AND  BUNCE 


Cranium. 


—    7  Cervical  Vertebras 


Clavicle. 
Scapula. 


12  Dorsal  Vertebras. 
Hi 


5  Lumbar  Vertebrne 


Ilium. 

Ulna. 

Kadius. 

Pelvis. 


Bones  of  the  Carpus. 

Bones  of  the  Meta- 
carpus. 

Phalanges  of  Fibers. 


Femur. 


Patella. 


Tibia. 
Fibula. 


Bones  of  the  Tarsus. 
Bones  of  th«  Meta- 
tarsus. 
Pholunge*  of  Toer 


THE   SKELETON  (»rr««  HOLDER). 


OUTLINES 


OF 


PHYSIOLOGY 


BY 

EDWARD  GROVES  JONES,  A.  B.,  M.  D, 

PROFESSOR   OF   SURGERY  IN   ATLANTA  SCHOOL  OF  MEDICINE 


AND 

ALLEN  H.  BUNCE,  A.  B.,  M.  D, 

ASSOCIATE  PROFESSOR   OF  PHYSIOLOGY  IN  THE 
ATLANTA  SCHOOL  OF  MEDICINE 


THIRD  EDITION,  REVISED 
111  ILLUSTRATIONS 


v   v.nii  li 
PHILADELPHIA 

P.  BLAKISTON'S  SON  &  CO. 

1012  WALNUT  STREET 
1912 


-.,.: 

.'.'  •  -      «.,*•• 


BIOLOGY 
LIBRARY 

D 


COPYRIGHT,  1912,  BY  P.  BLAKISTON'S  SON  &  Co. 


vrr^orj 


THE.  MAPLE*  PRE8S*YORK»  PA 


TO 

DOCTOR  WILLIAM  S.  KENDRICK 

PROFESSOR  OF  MEDICINE  IN  THE  ATLANTA  SCHOOL  OF  MEDICINE 
THESE  PAGES  ARE  AFFECTIONATELY  DEDICATED 


743555 


PREFACE  TO  THIRD  EDITION 


IN  revising  this  work  my  aim  has  been  to  fulfill  the  original 
purpose  of  the  book  in  so  far  as  possible.  Dr.  Jones  said 
in  the  preface  to  the  first  edition:  "This  volume  has  been 
prepared  with  a  view  of  presenting,  in  as  convenient  form 
as  possible,  the  essential  facts  of  modern  physiology  as  related 
to  the  practice  of  medicine."  This  object  has  been  kept  in 
mind  in  the  preparation  of  the  third  edition. 

In  this  edition  the  Introduction,  the  chapters  on  The  Cell, 
The  Elementary  Tissues  and  on  the  Blood  have  been  entirely 
rewritten  and  new  illustrations  added.  A  new  chapter,  with 
illustrations,  on  The  Physiological  Characteristics  of  Muscle,  has 
been  added.  The  chapter  on  Secretion  has  been  rearranged. 
New  subject  matter  has  been  added  to  the  chapter  on  The 
Physiology  of  Digestion  and  Absorption.  Other  minor  changes 
have  been  made  throughout  the  book  where  such  were  deemed 
advisable. 

Especial  acknowledgment  is  due  to  Dr.  Robert  G.  Stephens 
for  his  valuable  work  in  preparing  the  second  edition. 

In  the  preparation  of  this  edition,  no  claim  to  original  inves- 
tigation is  made.  I  have  freely  used  the  works  of  other  authors 
when  necessary. 

ATLANTA,  GA.  ALLEN  H.  BUNCE 


vn 


PREFACE  TO  FIRST  EDITION 


THIS  volume  has  been  prepared  with  the  view  of  presenting, 
in  as  convenient  form  as  possible,  the  essential  facts  of  modern 
physiology  as  related  to  the  practice  of  medicine.  In  the  exe- 
cution of  this  purpose  brevity  has  been  made  a  prime  considera- 
tion; therefore,  such  details  as  are  of  secondary  importance  are 
omitted,  theories  are  avoided,  and  conclusions  are  recorded 
without  argument.  There  is  no  short  road  to  knowledge,  and 
it  would  be  unfortunate  should  such  a  book  as  this  in  any  way 
discourage  extended  research ;  but  students  in  college  have  none 
too  much  time  to  devote  to  any  one  subject,  and  any  simple  col- 
lection of  pertinent  facts,  however  brief,  can,  if  reliable,  be  used 
to  great  advantage.  I  have  endeavored,  however,  to  make  the 
work  sufficiently  exhaustive  to  be  self-explanatory,  believing 
that  otherwise  economy  of  expression  is  practised  at  the  expense 
of  the  reader's  interest. 

A  maximum  of  space  has  been  given  to  those  subjects  which 
seem  of  most  practical  importance.  The  chemistry  of  the  body, 
the  special  senses  and  embryology  have  not  been  treated  in 
great  detail.  It  has  been  thought  undesirable  to  omit  a  brief 
anatomical  description  of  the  separate  organs  discussed. 

In  the  preparation  of  this  volume  no  claim  to  original  inves- 
tigation is  made.  The  writings  of  various  authorities  have  been 
freely  drawn  upon.  Especial  acknowledgment  is  due  to  the 
following  authors:  Ho  well  (American  Text-book),  Halliburton 
(Kirkes'  Handbook),  Flint,  Verworn  and  Stewart. 

I  am  under  obligations  to  Dr.  J.  Clarence  Johnson,  whose 
lectures  have  been  of  great  value  to  me,  and  to  Dr.  Frank  K. 
Boland,  who  has  written  the  whole  of  Chapter  II.,  read  the  proof 
sheets,  and  rendered  other  valuable  assistance  in  connection 
with  the  work. 

ATLANTA,  GA.  E.  G.  J. 

ix 


CONTENTS. 


PAGE 

INTRODUCTION       xv 

CHAPTER  I. 

THE  CELL i 

CHAPTER  II. 

THE  ELEMENTARY  TISSUES 7 

The  epithelial  tissues 7 

The  connective  tissues n 

The  muscular  tissues 19 

The  nervous  tissues 22 

CHAPTER  III. 

/ 

PHYSIOLOGICAL  CHARACTERISTICS  OF  MUSCLE 23 

CHAPTER  IV. 

SECRETION .  27 

Sebaceous  glands 30 

Mammary  glands 31 

Thyroid  glands 32 

Adrenal  glands 33 

Pituitary  body 34 

Testis  and  ovary 34 

CHAPTER  V. 

THE  BLOOD 35 

CHAPTER  VI. 

THE  CIRCULATION  OF  THE  BLOOD 41 

The  heart      42 

Circulation  in  blood-vessels 46 

xi 


Xll  CONTENTS 

PAGB 

Structure  of  the  blood-vessels 47 

The  lymph 57 

CHAPTER  VII. 

THE  PHYSIOLOGY  OF  DIGESTION  AND  ABSORPTION 63 

Foods 63 

Digestion 68 

Prehension 70 

Digestion  in  the  mouth 71 

The  salivary  glands  and  their  secretion 71 

Deglutition 78 

Digestion  and  absorption  in  the  stomach 81 

The  gastric  glands 85 

Digestion  and  absorption  in  the  intestines 96 

The  small  intestine      96 

The  large  intestine 117 

Absorption  in  general *    .    .    . 122 

Absorption  from  the  alimentary  canal 126 

CHAPTER  VIII. 

RESPIRATION 131 

Anatomy  of  the  respiratory  organs 132 

Mechanism  of  respiration 138 

CHAPTER  IX. 

NUTRITION,  DIETETICS  AND  ANIMAL  HEAT 171 

Nutrition 171 

Dietetics 181 

Animal  heat      184 

CHAPTER  X. 

EXCRETION 192 

The  kidneys 192 

The  skin 208 

CHAPTER  XI. 

THE  NERVOUS  SYSTEM 214 

The  cerebro-spinal  system 235 


CONTENTS  Xlll 

PAGE 

The  spinal  cord 237 

The  encephalon 251 

The  medulla  oblongata       251 

The  pons  varolii 255 

The  crura  cerebri 256 

The  cerebrum 260 

The  cerebellum 274 

The  cranial  nerves 275 

The  spinal  nerves 295 

The  sympathetic  system 297 

CHAPTER  XII. 

THE  SENSES 304 

Common  sensations 304 

Special  sensations 305 

The  sense  of  touch 305 

The  sense  of  smell 306 

The  sense  of  sight 307 

The  sense  of  taste 316 

The  sense  of  hearing 317 

The  production  of  the  voice 324 

CHAPTER  XIII. 

REPRODUCTION 327 

INDEX 365 


INTRODUCTION 


THE  science  which  treats  of  the  structure,  function  and  organi- 
zation of  living  forms,  both  vegetable  and  animal,  is  called 
biology.  That  branch  of  biology  which  describes  animal  life 
exclusively  is  termed  zoology,  while  that  branch  which  de- 
scribes vegetable  life  exclusively  is  termed  botany. 

The  study  of  the  form  of  organisms,  both  vegetable  and 
animal,  is  termed  morphology.  Morphology  is  further  divided 
into  (i)  histology,  which  treats  of  the  formed  elementary  con- 
stituents of  organisms,  and  (2)  anatomy,  which  treats  of  the 
parts  and  organs  of  the  organism. 

After  the  form  and  structure  of  an  organism  has  been  studied, 
the  next  step  is  the  study  of  the  work  which  the  organism  has 
to  perform.  This  study  of  the  vital  phenomena,  or  life,  of  the 
organism  is  called  physiology.  Physiology  may  be  either 
animal  or  vegetable.  Human  physiology  is  that  branch  of 
physiology  which  treats  of  the  vital  phenomena  occurring  in 
man. 

The  structural  unit  of  the  body  is  the  cell.  Myriads  of  cells 
are  grouped  together  to  form  organs.  An  organ  may  be  denned 
as  a  group  of  cells  combined  together  to  perform  some  special 
function,  e.  g.,  the  kidney  is  an  organ  whose  special  function  is 
the  secretion  of  urine.  The  organs  are  further  grouped  together 
to  form  systems.  Thus  we  have  the  circulatory  system  com- 
posed of  the  heart,  arteries,  veins,  and  capillaries.  Now  the 
study  of  the  function  which  the  circulatory  system  has  to  per- 
form is  the  physiology  of  circulation.  Likewise  we  may  sub- 
divide physiology  into  the  physiology  of  the  nervous  system, 

xv 


XVI  INTRODUCTION 

the  physiology  of  the  digestive  system,  the  physiology  of  the 
respiratory  system,  etc. 

Thus  we  have  seen  the  relation  physiology  bears  to  the  other 
sciences  dealing  with  life  and  the  special  part  human  physiology 
plays  in  the  whole  of  physiology.  In  the  following  pages  we 
have  endeavored  to  give  a  brief  outline  of  human  physiology. 


OUTLINES  OF  PHYSIOLOGY,:- 


CHAPTER  'I: •••'•'"••- 

',' 

THE  CELL. 

ALL  the  tissues  of  the  body  are  made  up  of  cells  and  inter- 
cellular substance.  All  the  cells  are  descended  from  one  parent 
cell,  called  the  ovum,  while  the  intercellular  substance  is  created 
through  the  medium  of  the  cells. 


Nuclear 
membrane. 


Linin. 


Nuclear  fluid, 
(matrix) . 


Nucleolus. 


Chromatin  cords 

(nuclear 

network). 


Nodal  enlarge- 
ments of  the 
chromatin. 


Cell  membrane. 
-V Exoplasm. 

Microsomes. 
Centrosome. 

^//  ^ '  ^J  J/r  7 Spongioplasm. 

Hyaloplasm. 

--==—--  Foreign  inclosures. 


FIG.  i. — Diagram  of  a  cell. 
Microsomes  and  spongioplasm  are  only  partly  drawn.     (Brubaker.) 

A  cell,  which  is  the  histologic  unit  of  the  body,  may  be  denned 
as  an  irregular  round  or  oval  mass  of  protoplasm  of  microscopic 
size,  enclosing  a  small  indistinct  spherical  body,  the  nucleus. 


2  THE    CELL 

The  essential  parts  of  the  cell  are,  (i)  the  cystoplasm,  whicn 
is  a  special  name  given  to  the  protoplasm  forming  the  cell-body 
and  (2)  the  nucleus,  which  is  a  small  round  or  oval  body 
embedded  in  the  cytoplasm.  A  great  many  cells  are  surrounded 
oy  a' cell-wall  or  cell- membrane,  but  this  cannot  be  regarded  as 
one  of-  the  essential  elements  since  all  cells  do  not  possess  such 
rnem'jr&nes. 

(1)  The   Cytoplasm. — This  is  a  gelatinous  or   semi-fluid, 
granular  substance,  transparent  and  generally  colorless.     Chem- 
ically it  consists  of  water  and  salts,  together  with  various  organic 
substances,  called  proteids,  which  are  complex  combinations 
of   carbon,   hydrogen,   oxygen   and   nitrogen,    and   sometimes 
phosphorus  and  sulphur.     The  proteids  of  the  cytoplasm  con- 
tain little  phosphorus,  while  those  of  the  nucleus  are  rich  in  it. 

The  cytoplasm  does  not  always  present  the  same  structural 
appearance  since  its  constituents  vary  in  their  condition  and 
arrangement.  In  some  cells  it  has  a  clear  homogeneous  appear- 
ance, while  in  others  it  contains  fine  spherical  particles  which 
give  it  a  granular  structure.  When  these  granules  are  large 
and  clear,  and  are  surrounded  by  denser  areas  they  give  to  the 
cytoplasm  an  alveolar  outline.  But  most  frequently  the  cyto- 
plasm contains  in  its  structure  a  meshwork  of  threads  or  fibrils 
which  give  it  a  reticular  appearance.  This  network  of  fibrils 
is  called  the  spongioplasm  which  encloses  a  less  firm  portion, 
the  hyaloplasm  (Fig.  i). 

However,  in  all  these  varieties,  the  cytoplasm  has  both  an 
active  and  a  passive  structure.  In  young  granular  cells  the 
active  substance  is  represented  by  small  spherical  particles, 
called  microsomes  (Fig.  i).  These  are  not  always  evenly  dis- 
tributed throughout  the  cytoplasm,  but  are  grouped  in  an  area 
near  the  nucleus,  while  the  area  next  the  cell-wall  is  almost  free 
from  granules.  The  dense  inner  area  is  called  the  endoplasm, 
while  the  clear  outer  area  is  called  the  exoplasm  (Fig.  i). 

(2)  The  nucleus,  which  is  the  second  essential  part  of  a 


THE    VITAL   PHENOMENA    OF    CELLS  3 

typical  cell,  is  a  small  round  or  oval  body  contained  within  the 
cell.  It  is  usually  surrounded  by  a  distinct  nuclear  membrane, 
except  during  division.  Nuclei  play  an  important  role  in  cell 
reproduction  and  cell  nutrition.  They  are  characterized  by 
their  affinity  for  certain  stains,  e.  g.,  hematoxylin. 

The  substance  of  the  nucleus,  the  karyoplasm,  may  be  divided 
into  two  parts — the  nuclear  fibrils  which  form  an  irregular  reticu- 
lum,  and  the  nuclear  matrix  which  forms  the  intervening  semi- 
fluid mass.  The  nuclear  fibrils,  when  properly  stained,  are 
found  to  consist  of  minute  irregular  masses  of  a  deeply  colored 
substance,  called  chromatin,  in  recognition  of  their  affinity 
for  certain  stains.  The  chromatin  particles  are  supported 
within  delicate  and  colorless  threads  of  linin.  The  nuclear 
matrix,  which  is  semi-fluid  in  character  and  which  occupies  the 
spaces  between  the  nuclear  fibrils,  possesses  a  very  weak  affinity 
for  the  stains  used  to  color  the  chromatin.  Hence,  it  usually 
appears  clear  and  untinted.  Chemically,  the  chromatin  con- 
tains a  substance  nuclein,  which  is  rich  in  phosphorus. 

The  nucleolus  ordinarily  appears  as  a  small  spherical  mass 
among  the  nuclear  fibrils.  It  is  supposed  to  be  of  little  signifi- 
cance in  so  far  as  the  vital  phenomena  of  the  cell  are  concerned. 

The  Vital  Phenomena  of  Cells. — The  vital  phenomena  of 
the  cell  include  all  those  processes  and  changes  which  it  under- 
goes during  its  life  and  which  take  place  in  the  performance  of 
its  various  functions.  They  include  (i)  metabolism,  (2)  growth, 
(3)  reproduction  and  (4)  irritability. 

(i)  Metabolism  includes  all  those  processes  by  which  the  cell 
is  enabled  to  select  from  the  various  substances  furnished  it  and 
convert  them  into  its  own  substance,  and,  secondly,  those  proc- 
esses whereby  the  cell  is  enabled  to  cast  off  the  waste  products 
set  free  by  its  activity.  The  first  process,  that  by  which  the 
cell  takes  the  simple  substances  furnished  it  and  converts  them 
into  its  complex  compounds,  is  called  anabolism,  or  construc- 
tive metabolism. 


4  THE    CELL 

The  process  whereby  the  cell  breaks  up  these  complex  com- 
pounds formed  by  anabolism  and  discharges  them  from  its 
substance,  is  called  katabolism,  or  destructive  metabolism.  A 
good  example  of  anabolism  is  that  by  which  the  vegetable  cells 
take  such  substances  as  carbon  dioxide,  water  and  inorganic 
salts  and  prepare  food-material  for  the  nutritive  and  katabolic 
processes  in  animals. 

(2)  Growth  is  the  natural  sequence  of  the  nutritive  changes 
effected  by  metabolism  and  may  be  unrestricted  and  equal  in 
all  directions.     However,  this  is  not  usually  the  case  as  is  shown 
by  the  fact  that  cells  are  so  intimately  associated  with  other 
structural  elements  as  to  influence  and  modify  their  growth. 
These  result  in  unequal  growth,  to  which  the  specialization  of 
cells  is  due.     Examples  of  the  unequal  growth  of   cells  are 
shown  in  the  columnar  cells  of  epithelium,   the  neurones  of 
nervous  tissue,  the  fibers  of  muscle  tissue,  etc. 

(3)  Reproduction   may   be    regarded    as   the    culmination   of 
the  activities  of  the  cell,  for  by  this  process  the  cell  loses  its 
individuality  and  continues  its  life  in  that  of  its  offsprings. 
There  are  two   methods  by  which  cells  may  reproduce  them- 
selves, (i)  by  direct  cell  division  or  amitosis  and  (2)  by  indirect 
cell  division  or  mitosis. 

(4)  Irritability  is  that  property  of  cells  whereby  they  are 
enabled  to  respond  to  stimuli,  i.  £.,  to  change  their  form  and 
shape  in  response  to  these  stimuli.     The  various  stimuli  which 
affect  cells  may  be  mechanical,  thermal,  nervous,  chemical  or 
electrical. 

Cell  Division. — (i)  Direct  cell  division,  or  amitosis,  is  the 
simplest  form  of  cell  division.  In  this  form  of  cell  division  the 
nucleus  and  protoplasm  constrict  in  the  middle  until  two  new 
cells  are  formed.  This  form  of  cell  division  does  not  occur  in 
the  higher  animals  except  as  a  secondary  process. 

(2)  In  the  higher  animals  cell  division  takes  place  chiefly 
by  the  indirect  or  mitotic  method.  This  may  be  described 


CELL    DIVISION 


briefly  as  follows:  In  the  beginning  of  this  phenomenon,  the 
nucleus,  which  plays  the  most  important  role,  grows  larger. 
Its  chromatin  greatly  increases  and  becomes  contorted  so  as  to 
form  a  dense  convolution,  the  close  skein,  or  spireme.  Then 
the  chromatin  fibrils  further  thicken,  become  less  convoluted 
and  form  irregularly  arranged  loops,  the  loose  skein.  During 

Close  Skein  Mother  stars 

(viewed  from  the                       Loose  Skein  (viewed  from 

side) ;  Polar  field,  (viewed  from  above — i.  e.,  from  the  the  side). 

pole). 


Mother  Star         Daughter  Star 
(viewed  from  above). 


Beginning.  Completed. 

Division  of  the  Protoplasm. 


FIG.  2. — Karyokinetic  figures  observed  in  the  epithelium  of  the  oral  cavity 
of  a  salamander. 

The  picture  in  the  upper  right-hand  corner  is  from  a  section  through  a  dividing 
egg  of  Siredon  pisciformis.  Neither  the  centrospmes  nor  the  first  stages  of  the  de- 
velopment of  the  spindle  can  be  seen  by  this  magnification.  X  560.  (From  Brubaker.} 

the  formation  of  these  skeins  the  nuclear  membrane  and  the 
nucleoli  disappear.  The  fibrils  of  the  loose  skein  now  separate 
at  their  peripheral  turns  into  a  score  of  loops,  the  closed  ends  of 
which  point  toward  a  common  center — a  clear  space  called  the 
polar  field.  When  seen  from  above  these  loops  of  chromatin 
make  a  wreath,  called  the  mother  wreath;  when  seen  from  the 


6  THE    CELL 

side,  they  make  a  star,  called  the  mother  star  or  aster.  While 
the  loose  skeins  are  forming,  delicate  striae  appear  within  the 
achromatin,  so  disposed  as  to  make  their  bases  within  the  polar 
field  and  directed  toward  one  another,  and  their  apices  directed 
toward  the  future  new  nuclei.  These  achromatin  figures  con- 
stitute the  nuclear  spindle.  They  then  arrange  themselves  into 
two  daughter  wreaths,  or  asters,  similar  to  the  mother  star. 
At  this  juncture  the  cell  protoplasm  begins  to  divide  by  becom- 
ing constricted  in  the  center.  The  daughter  stars  are  converted 
into  two  new  nuclei,  in  the  inverse  order  to  that  by  which  the 
original  nucleus  was  broken  up.  Nuclear  membranes  and 
nucleoli  appear,  the  cell  protoplasm  divides  into  two  new  cells, 
and  the  cycle  is  completed. 

Derivation  of  Tissues. — The  primary  parent  cell  divides 
into  an  innumerable  mass  of  cells,  which  is  called  the  blasto- 
derm. The  blastoderm  soon  divides  into  two  more  or  less 
distinct  layers,  an  outer  and  an  inner,  named  ectoderm  and 
entoderm,  between  which  a  middle  layer  develops,  the  meso- 
derm.  From  these  three  primary  layers  all  of  the  various  tissues 
of  the  body  are  later  developed.  (See  Embryology.) 


CHAPTER  II. 
THE  ELEMENTARY  TISSUES. 

THE  tissues  which  make  up  the  various  organs  and  parts 
of  the  body  may  be  divided  into  the  following  groups:  (i)  Epi- 
thelial tissue,  (2)  connective  tissue,  (3)  muscular  tissue  and  (4) 
nervous  tissue. 

The  Epithelial  Tissues. 

The  epithelial  tissues  include  those  which  form  the  covering 
for  the  body,  the  lining  of  the  digestive  canal,  the  respiratory 
tract  and  the  genito-urinary  tract.  They  also  constitute  the 
derivatives  of  the  epidermis,  such  as  nails,  hair,  sebaceous 
glands,  and  the  lining  of  the  glands  connected  with  the  digestive 
and  genito-urinary  systems. 

These  tissues  are  composed  of  cellular  and  intercellular 
elements  and  perform  various  functions  in  different  parts  of  the 
body.  In  the  skin  where  they  constitute  the  epidermis,  they 
protect  the  delicate  surface  of  the  true  skin  beneath;  in  the 
alimentary  and  genito-urinary  canals  they  aid  in  secretion  and 
excretion;  in  the  respiratory  system  they  preserve  an  equable 
temperature,  while  in  all  internal  parts  they  yield  lubricants. 

These  tissues  are  characterized  by  the  preponderance  of  the 
cellular  over  the  intercellular  elements.  The  intercellular 
structure  consists  of  a  cement  substance  which  holds  the  cells 
together  and  through  which  the  food  for  the  cells  is  absorbed. 
They  contain  no  blood-vessels  and  no  nerves.  The  tissue 
usually  rests  upon  a  basement  membrana,  or  membrana  propria, 
which  is  a  modification  of  the  connective  tissue  beneath. 


8 


THE    ELEMENTARY    TISSUES 


Varieties. — The  varieties  of  epithelium  may  be  classed  as 
follows:  (i)  Squamous,  (a)  simple,  consisting  of  a  single  layer, 
(b)  stratified,  consisting  of  several  layers;  (2)  Columnar,  (a) 
simple,  (b)  stratified;  (3)  Modified,  (a)  ciliated,  (b)  gob  et,  (c) 
pigmented,  (d)  glandular,  (e)  neuro-epithelium. 

(i)  Squamous  Epithelium.— (a)  Simple  squamous  epithe- 
lium consists  of  a  single  layer  of  cells  which,  when  viewed  from 


FIG.  3. — From  a  section  of  the  lung  of  a  cat,  stained  with  silver  nitrate. 

N,  Alveoli  or  air-cells,  lined  with  large,  flat,  nucleated  cells,  with  some  smaller 
polyhedral  nucleated  cells.  (Halliburton  after  Klein  and  Noble  Smith.) 

above,  appear  as  flattened  polyhedral  nucleated  plates  like  a 
regular  mosaic.  It  occurs  in  but  a  few  places,  lining  the  air 
sacs  of  the  lungs,  the  mastoid  cells,  the  membranous  labyrinth 
and  crystalline  lens  (Fig.  3). 

(b)  Stratified  squamous  epithelium  is  composed  of  several 
layers  of  epithelial  cells  placed  upon  one  another.  The  deepest 
layer,  which  rests  upon  the  basement  membrane,  is  composed 
of  irregularly  columnar  cells  which  have  their  nuclei  near  the 


EPITHELIAL    TISSUES  9 

lower  border  of  the  cells.  As  they  approach  the  surface 
the  layers  become  flatter  and  more  scale-like  and  possess  less 
vitality.  As  the  outer  layers  are  worn  away  the  lower  more 
vigorous  layers  push  upward  to  the  surface  to  take  their  place. 
In  the  middle  strata,  where  the  cells  are  polyhedral  in  shape, 
we  find  the  layer  of  prickle  cells  which  have  minute  projecting 


FIG.  4. — Vertical  section  of  the  stratified  epithelium  of  the  rabbit's  cornea. 

a,  anterior  epithelium,  showing  the  different  shapes  of  the  cells  at  various  denths 

from  the  free  surface;  b,  a  portion  of  the  substance  of  cornea.     (Kirkes  after  Klein.) 

spines  by  which  they  are  connected  together.  This  layer  is 
sometimes  called  the  stratum  spinosum.  Just  below  this  is  the 
stratum  germinativum,  or  germinating  layer. 

(2)  Columnar  Epithelium. — (a)  The  simple  columnar  va- 
riety consists  of  a  single  layer  of  column  or  rod-shaped  cells,  set 
upright,    longitudinally    striated    and    containing    oval-shaped 
nuclei.     This  variety  is  found  in  the  lining  of  the  stomach  and 
intestines. 

(b)  In  the  stratified  columnar  variety  the  single  layer  of  cells 
is  replaced  by  several  layers  and  the  superficial  elements  alone 
are  typical.  This  type  is  found  in  the  vas  deferens.  Ciliated, 
it  occurs  in  the  Eustachian  tube,  lachrymal  ducts,  respiratory 
part  of  the  nasal  fossae,  ventricle  of  larynx,  trachea  and  bronchi, 
epididymis  and  first  part  of  vas  deferens. 

(3)  Modified  Epithelium. — (a)  Ciliated  epithelium  is  more 
common  with  the  columnar  variety  than  with  any  other.     Each 


10 


THE    ELEMENTARY    TISSUES 


of  the  ciliated  epithelial  cells  presents  on  its  free  surface  twenty 
or  more  small,  hair-like,  protoplasmic  appendages,  called  cilia. 
During  life  these  small  processes  are  in  constant  rapid  motion, 
waving  in  a  direction  toward  the  outlet  of  the  cavity  in  which 


FIG.  5. — Ciliated  epithelium  of  the  human  trachea. 

a,  layer  of  longitudinally  arranged  elastic  fibers;  b,  basement  membrane;  c,  deep- 
est cells,  circular  in  form;  d,  intermediate  elongated  cells;  e,  outermost  layer  of  cells 
fully  developed  and  bearing  cilia.  Xsso.  (Kirkes  after  Kolliker.) 

they  are  found.  In  the  genital  organs  they  are  important  in 
bringing  together  the  male  and  female  elements  of  reproduction, 
while  in  the  respiratory  tract  they  are  concerned  in  aiding  the 
passage  of  the  mucus  and  in  the  expul- 
sion of  foreign  bodies. 

(b)  Goblet  cells  are  found  on  all  sur- 
faces covered  by  columnar  epithelium, 
but   especially   in   the  large  intestine. 
They  secrete  mucin,  the  main  constit- 
uent of  mucus,  which  so  distends  the 
cell  that  it  ultimately  bursts  and  sets 
free  its  contents. 

(c)  Pigmented  epithelium  is  ordinary 
epithelium,  the  protoplasm  of  which  has  become  invaded  and 
colored  by  foreign  matter,  such  as  fat,  proteid,  etc.     Such  cells 
are  constant  in  the  deeper  layers  of  the  epidermis,  especially  of 


FIG.  6.— Goblet  cells. 
(Halliburton  after  Klein.} 


CONNECTIVE    TISSUES  II 

certain  races,  as  the  negro.     It  is  also  found  in  the  choroid  coat 
of  the  eye. 

(d)  Glandular  epithelium  may  be  columnar,  spherical  or  poly- 
hedral in  shape.     It  is  found  lining  the  terminal  recesses  of 
secreting  glands.     The  protoplasm  of  the  cell  usually  contains 
the  material  which  the  gland  secretes. 

(e)  N euro -epithelium  is  the  name  given  to  that  covering  those 
parts  toward  which  the  nerves  of  special  sense  are  directed,,  and 
is  epithelium  of  the  highest  specialization.     It  occurs  in  the 
retina,  the  membranous  labyrinth  and  in  the  olfactory  and  taste 
cells. 

The  Connective  Tissues. 

All  these  tissues  are  developed  from  the  same  embryonal 
elements,  but  present  varieties  differing  widely  in  appearance 
and  properties.  They  are  characterized  by  the  preponderance 
of  the  inter-cellular  over  the  cellular  elements.  The  physical 
characteristics  of  these  tissues  are  very  important  and  depend 
mostly  upon  the  intercellular  elements.  Their  purpose  in  the 
animal  economy  is  to  furnish  a  supporting  and  connecting 
framework  for  the  body.  In  the  embryonal  state  the  inter- 
cellular substance  is  semi-fluid  and  gelatinous.  Later,  in  adult 
connective  tissue  it  becomes  more  definitely  formed,  although 
it  is  still  soft.  In  adult  areolar  tissue  the  intercellular  substance 
becomes  tough  and  yielding.  When  this  intercellular  sub- 
stance becomes  impregnated  with  calcareous  salts  we  have 
bone.  However,  during  all  these  changes  in  the  intercellular 
substance  little  or  no  change  has  taken  place  in  the  cellular 
structure — the  bone-corpuscle,  the  cartilage-cell,  the  tendon- 
cell  and  the  connective-tissue  cell  all  being  essentially  identical. 

The  divisions  of  connective  tissue  are:  (i)  Mucous  Tissue, 
(2)  Reticular  Tissue,  (3)  Fibrous  Tissue,  (4)  Adipose  Tissue, 
(5)  Cartilage,  (6)  Bone. 


12 


THE    ELEMENTARY    TISSUES 


(i)  Mucous  Tissue.— This  is  the  most  immature  form  of 
connective  tissue  and  consists  of  a  loose  protoplasmic  network 
having  a  gelatinous  intercellular  substance.  It  is  found  in 


FIG.  7. — Tissue  of  the  jelly  of  Wharton  from  umbilical  cord. 

a,  Connective-tissue  corpuscles;  b,  fasciculi  of  connective-tissue  fibers;  c,  spherical 
cells.      (Halliburton  after  Frey.) 


FIG.  8. — Retiform  tissue  from  a  lymphatic   gland,   from  a   section   which 
has  been  treated  with  dilute  potash.     (Halliburton  after  Schafer.} 

Wharton's  jelly  in  the  embryo  and  in  certain  tumors,  known 
as  myxoma. 

(2)  Reticular  Tissue. — This  is  composed  chiefly  of  a  net- 
work of  connective-tissue  cells  which  enclose  a  mass  of  lymphoid 


CONNECTIVE    TISSUES  13 

elements.  It  forms  the  connecting  layer  beneath  the  skin,  the 
submucous  and  subserous  tissues,  and  the  layer  between  the 
muscles.  It  receives  its  name  on  account  of  the  areolae  or 
spaces  within  its  substance,  which  admit  the  adjacent  parts  to 
move  easily  upon  one  another.  It  consists  of  white  and  yellow 
fibers  in  about  an  equal  proportion. 

(3)  Fibrous  Tissue. — This  variety  includes  all  the  more 
usual  forms  of  connective  tissue  found  in  the  various  parts  of 
the  body.  It  may  be  further  subdivided  into:  (a)  White  fibrous 
tissue,  (b)  yellow  elastic  and  (3)  loose  fibrous  or  areolar  tissue. 


FIG.  9. — Bundles  of  the  white  fibers  or  areolar   tissue   partly   unravelled. 
(Kirkes  after  Sharpey.} 

(a)  White  fibrous  tissue  is  composed  of  groups  or  bundles  of 
fibers  which  have  a  wavy  longitudinal  striation.  It  is  tough 
and  inelastic  and  forms  ligaments,  tendons  and  membranes  in 
various  parts  of  the  body.  Chemically  this  tissue  is  composed 
of  a  complex  albuminoid  substance,  collagen.  Upon  being 
treated  with  acetic  acid  the  fibers  become  swollen  and  trans- 
parent and  finally  invisible. 


14  THE    ELEMENTARY    TISSUES 

(b)  Yellow  elastic  tissue  is  composed  of  bundles  of  long,  regu- 
lar and  branched  fibers.  It  is  characterized  by  its  marked 
elasticity.  It  is  found  in  the  vocal  cords,  longitudinal  coat  of 
the  trachea  and  bronchi,  inner  coat  of  blood-vessels,  especially 
the  large  arteries,  and  in  some  ligaments.  Its  yellow-tinted 
fibers  are  seen  in  parallel  waves  and  are  larger  than  those  in 
the  white  tissue.  They  sometimes  form  a  web-like  layer,  as  in 
the  fenestrated  layer  of  Henle  in  the  arteries. 


FIG.  10. — Elastic  fibers  from 
the  ligamenta  subflava.  Xaoo. 
(Halliburton  after  Sharpey.) 


FIG.  1 1 . — Group  of  fat-cells  (F  c)  with 
capillary  vessels  (c).  (Kirkes  after 
Noble  Smith.) 


(4)  Adipose  Tissue. — This  tissue  exists  in  nearly  all  parts 
of  the  body  except  the  subcutaneous  tissue  of  the  eyelids,  the 
penis  and  scrotum,  the  nymphae  and  in  certain  parts  of  the 
lungs.  It  is  nearly  always  found  within  the  meshes  of  areolar 
tissue,  where  it  forms  lobules  of  fat.  Fatty  matter  in  the  form 
of  oily  tissue  is  found  in  the  brain,  liver,  blood  and  chyle.  The 
tissue  is  densest  beneath  the  skin,  especially  of  the  abdomen, 


CONNECTIVE    TISSUES 


around  the  kidneys,  between  the  furrows  on  the  surface  of  the 
heart  and  in  bone  marrow.  It  has  a  rich  blood  supply. 

(5)  Cartilage.— Those  tissues  in  which  the  intercellular 
substance  has  undergone  condensation  until  it  appears  homo- 
geneous are  classified  as  cartilage.  Consequent  upon  the 
differences  exhibited  by  the  intercellular  matrix  it  is  divided 
into  the  following  varieties:  (a) 
Hyaline,  (b)  elastic  and  (c)  fibrous. 

(a)  Hyaline  cartilage  is  of  firm 
consistence,  considerable  elasticity 
and  is  pearly  blue  in  color.     It  is 
enveloped  in  a  fibrous  membrane, 
the  perichondrium,  from  the  vessels 
of  which  it  derives  its  nutrition.     It 
is  composed  of  cells,  irregular  in 
outline  and  arranged  in  patches  of 
various  shapes,  which  are  embedded 
in  a   homogeneous   matrix.      The 
articular   surfaces    of    bones,    the 
costal    cartilages,    and   the    larger 
cartilages  of  the  larynx,  trachea  and 
bronchi,  and,  also,  those  of  the  nose 

and  Eustachian  tube  are  formed  of  this  variety.  In  the  em- 
bryo this  cartilage  forms  nearly  the  whole  of  the  future  bony 
skeleton. 

(b)  Elastic  cartilage  is  characterized  by  the  presence  of  an 
abundance   of   elastic  fibers  in  the   matrix.     These   resemble 
those  found  in  the  yellow  variety  of  elastic  tissue.     This  variety 
of  cartilage  is  found  in  the  external  ear,  epiglottis,  cornicula 
laryngis  and  Eustachian  tube. 

(c)  Fibrous  cartilage  is  characterized  by  the  presence  of  a 
large  amount  of  white  fibrous  tissue  in  the  matrix.     It  combines 
the  toughness  and  flexibility  of  fibrous  tissue  with  the  firmness 
and  elasticity  of  cartilage.     It  is  found  chiefly  in  the  interver- 


FIG.  12. — Section  of  hyaline 
cartilage. 

From  the  end  of  a  growing  bone, 
showing  a  decrease  in  the  intercel- 
lular substance  compared  with  the 
number  of  cell-elements,  which  are 
arranged  in  rows.  (Yeo.) 


i6 


THE    ELEMENTARY    TISSUES 


tebral  disks,  the  symphyses  and  interarticular  disks  of  certain 
joints,  and  lining  bony  grooves  for  tendons. 

Chemically,  cartilage  is  complex,  consisting  of  a  mixture  of 


FIG.  13. — Elastic  fibro-cartilage, 
Showing  cells  in  capsules  and  elastic  fibers  in  matrix.      (From  Yeo  after  Cadiat.) 

collagen,  chondro-mucoid  and  albuminoid  substances.  On 
boiling,  it  yields  a  substance  known  as  chondrin,  which  on 
cooling  turns  to  gelatin. 


FIG.  14. — White  fibro-cartilage, 
Showing  cells,  a,  in  capsules  and  fibrillar  matrix,  b.     (Yeo  after  Cadiat.) 

(6)  Bone. — Bone  is  a  dense  form  of  connective  tissue  con- 
stituting the  skeleton  or  framework  of  the  body.     It  serves  to 


CONNECTIVE   TISSUES  17 

protect  vital  organs  in  the  skull  and  trunk  and  acts  as  levers 
which  are  worked  by  the  muscles  in  the  limbs.  The  tissue  is 
characterized  by  the  deposit  of  calcareous  or  lime  salts  within 
its  intercellular  substance,  to  which  its  well-known  hardness  is 
due.  Most  bones  may  be  divided  into  an  outer  layer  of  compact 
bone  and  an  inner  layer  of  spongy  or  cancellated  bone. 


FIG.  15. — Transverse  section  of  compact  bony  tissue  (of  humerus). 

Three  of  the  Hayersian  canals  are  seen,  with  their  concentric  rings;  also  the  lacunae, 
with  the  canaliculi  extending  from  them  across  the  direction  of  the  lamellae.  The 
Haversian  apertures  were  filled  with  air  and  debris  in  grinding  d9\vn  the  section,  and 
therefore  appear  black  in  the  figure,  which  reoresents  the  object  as  viewed  with 
transmitted  light.  The  Haversian  systems  are  so  closely  nacked  in  this  section  that 
scarcely  any  interstitial  lamellae  are  visible.  X  150.  (Kirkes  after  Sharpey.) 


Microscopically  bone  is  seen  to  consist  of  numbers  of  osseous 
layers  or  lamellae,  arranged  as,  (a)  circumferential  lamella  which 
are  arranged  parallel  to  the  inner  and  outer  surfaces  of  the  bone, 
(b)  Haversian  lamellae  which  are  arranged  concentrically  around 
the  Haversian  canals  and  (c)  interstitial  lamellae,  which  are 
arranged  irregularly  so  as  to  fill  in  the  spaces  which  the  other 


l8  THE    ELEMENTARY   TISSUES 

lamellae  do  not  fill.  The  Haversian  canals  are  minute  longi- 
tudinal channels,  each  surrounded  by  its  lamellae  within  which 
run  still  smaller  longitudinal  channels,  called  lacunae.  Connect- 
ing the  main  channel  and  the  lacunae,  and  radiating  in  all  direc- 
tions between  them  are  other  very  minute  channels  known  as 
canaliculi.  Each  Haversian  canal  with  its  surrounding  lamellae, 
lacunae  and  canaliculi  composes  an  Haversian  system. 

A  fibrous  membrane,  the  periosteum,  forms  the  outer  covering 
of  all  bones  except  when  they  are  covered  with  cartilage.  It 
consists  of  two  layers,  an  outer  fibrous  and  an  inner  fibro-elastic 
layer.  However,  during  the  period  of  development  a  third 
layer,  the  osteogenetic  layer,  lies  to  the  interior.  It  possesses 
a  rich  blood  supply  which  nourishes  the  subjacent  bone,  and 
contains  cells  which  later  become  bone-forming  elements — the 
osteoblasts. 

Bone  marrow  is  the  highly  vascular  substance  found  within 
the  central  cavity  of  the  long  bones  and  the  Haversian  canals. 
It  may  be  divided  into  two  classes:  (i)  Red  bone  marrow  and 
(2)  yellow  marrow.  In  early  childhood  all  the  marrow  in  the 
bones  is  red  or  has  a  reddish  tint,  but  in  adult  life  we  find  two 
kinds — the  red  and  the  yellow. 

(i)  Red  bone  marrow  is  classed  as  one  of  the  blood-forming 
organs  since  it  plays  an  important  role  in  the  formation  of  the 
blood.  When  stained  and  examined  under  the  microscope 
it  is  found  to  consist  of  a  delicate  connective-tissue  reticulum 
which  supports  the  blood-vessels  and  contains  in  its  meshes 
numerous  cells.  On  the  outside  of  the  marrow,  next  to  the  bone, 
we  find  a  thin  fibrous-tissue  coat,  the  endosteum,  which  lines 
the  medullary  cavity  and  extends  into  the  larger  Haversian 
canals.  The  more  numerous  of  the  cells  found  in  the  red 
marrow  are,  (a)  the  myelocytes,  which  are  very  numerous  and 
contain  several  different  varieties  of  cells,  (b)  the  eosinophiles, 
which  are  few  in  number,  but  which  are  conspicuous  by  the 
presence  of  coarse  granules  within  the  cytoplasm,  which  are 


MUSCULAR   TISSUES 


colored  intensely  by  acid  stains,  such  as 
eosin,  (c)  the  giant  cells,  which  are  very 
large,  but  contain  only  one  nucleus,  (d) 
the  erythroblasts,  which  are  nucleated  red 
blood-cells.  In  addition  to  these,  the 
red  marrow  contains  mast-cells,  fat- 
cells  and  osteoclasts,  or  multinuclear 
giant-cells. 

(2)  Yellow  bone  marrow  is  formed 
from  red  marrow  by  the  infiltration  of 
fat-cells  which  convert  it  into  adipose 
tissue.  When  examined  in  section 
yellow  marrow  resembles  ordinary  fat- 
tissue,  consisting  chiefly  of  large  com- 
pressed spherical  fat-cells  which  are 
supported  by  a  reticulum  of  connective 
tissue.  Yellow  marrow  is  found  in  all 
the  adult  long  bones,  except  at  their 
extremities. 

The  Muscular  Tissues. 

The  chief  characteristic  of  muscular 
tissue  which  distinguishes  it  from  all 
other  tissues  is  its  marked  contractility. 
This  variety  of  tissue  may  be  divided 
into  three  large  groups:  (i)  Striated 
muscle,  (2)  cardiac  muscle  and  (3)  smooth 
muscle. 

(i)  Striated  or  voluntary  muscle 
makes  up  the  greater  part  of  all  the 
skeletal  muscles  by  means  of  which  all 
voluntary  movements  are  made.  In 
addition  to  this,  it  constitutes  the  walls 


FIG.  16. — Two  fibers  of 
striated  muscle, 

In  which  the  contractile 
substance,  m,  has  been  rup- 
tured and  separated  from  the 
sarcolemma,  a  and  s;  p,  space 
under  sarcolemma.  (From 
Yeo  after  Ranvier.) 

of  the  abdomen,  and  a 


20  THE    ELEMENTARY    TISSUES 

few  of  the  muscles  connected  with  the  middle  ear,  tongue, 
pharynx,  larynx,  diaphragm,  and  generative  organs.  This 
group  of  muscular  tissue  is  composed  of  bundles  of  fibers, 
each  fiber  of  which  is  derived  from  a  single  cell  which  has 
many  nuclei.  Each  fiber  is  enclosed  in  a  thin,  homogeneous, 
elastic  membrane,  the  sarcolemma.  The  fibers  are  composed 
of  a  semi-fluid  and  viscous  material  which  is  called  the  muscle 
plasma.  The  muscle  plasma  consists  of  two  elements,  the  fibrils 


FIG.  17. — Striated  muscular  tissue  of  the  heart, 

Showing  the  trelliswork  formed  by  the  short  branching  cells,  with  central   nuclei. 

(Y**,) 

and  the  sarcoplasm.  The  fibrils  which  are  long  and  thread- 
like, running  the  entire  length  of  the  fiber,  consist  of  alternating 
light  and  dark  segments  which  fall  together  in  the  different 
fibrils  and  give  the  muscle  its  characteristic  striated  appear- 
ance. The  sarcoplasm,  which  varies  greatly  in  the  striated 
muscle  of  different  animals,  fills  in  the  space  between  the  fibrils. 
From  a  study  of  comparative  physiology  it  is  assumed  that  the 
fibrils  are  the  contractile  element  of  the  muscle  fiber,  while  the 
sarcoplasm  serves  a  general  nutritive  function. 


MUSCULAR    TISSUES 


21 


Striated  muscular  tissue  is  very  richly 
supplied  with  blood-vessels.  The  larger 
arteries  and  their  accompanying  veins 
enter  the  muscle  along  connective  tissue 
septa  and  then  break  up  into  smaller^ 
branches  and,  finally,  into  a  capillary 
network  which  supplies  the  individual 
muscle  fibers.  They  are  also  supplied 
with  lymphatics  which  occupy  the  clefts 
in  the  connective-tissue  septa  around  the 
fibers.  There  are  also  definite  lymph- 
vessels  which  accompany  the  blood- 
vessels within  the  muscle.  This  tissue 
is  also  supplied  with  both  motor  and 
sensory  nerves,  by  means  of  which  the 
stimuli  are  carried  to  and  from  the  mus- 
cle fibers. 

(2)  Heart  muscle  occupies  an  inter- 
mediate  position   between   the  striated 
voluntary  muscle  and  the  non-striated 
involuntary  muscle  tissue.     It  is  charac- 
teristic in  that  it  is  striated  and  invol- 
untary.    The  following  is  a  brief  sum- 
mary of  its  chief  distinguishing  features: 

(1)  Its  fibers  are  united  with  each  other 
at  frequent  intervals  by  short  branches, 

(2)  its  fibers  are  smaller  and  their  stria- 
tion  is  less  marked  than  in  voluntary 
muscle,   (3)  it  has  no  sarcolemma,  and 
(4)    its   nuclei  are  situated  within  the 
substance  of  the  fiber  and  not  upon  it. 

(3)  Smooth  or   involuntary  muscle 
occurs  in  bundles  and  thin  sheets  chiefly 
in  viscera  and  blood-vessels.     Its  general 


FIG.  18. — Cells  of  smooth 
muscle-tissue  from  the 
intestinal  tract  of  rabbit. 
(P'rom  Yeo  after  Ranvier.} 

A  and  B,  muscle-cells  in 
which  differentiation  of  the 
protoplasm  can  be  well  seen. 


22  THE    ELEMENTARY    TISSUES 

distribution  may  be  outlined  as  follows:  (i)  It  is  found  in  the 
digestive  tract  from  the  middle  of  the  esophagus  to  the  anus. 
(2)  in  the  capsule  of  the  pelvis  of  the  kidney,  (3)  in  the  trachea 
and  bronchi,  (4)  in  the  ducts  of  glands,  (5)  in  the  gall-bladder, 
(6)  in  the  vas  deferens  and  seminal  vesicles  of  the  male  repro- 
ductive organs,  (7)  in  the  uterus,  vagina  and  oviducts  of  the 
female  reproductive  organs,  (8)  in  the  blood-vessels  and  lym- 
phatics, (9)  in  the  iris,  ciliary  bodies  and  eye-lids  and  (10)  in 
the  hair  follicles,  sweat  glands,  and  skin  of  the  scrotum  and  in 
some  other  places  throughout  the  body. 

The  structural  unit  of  smooth  muscle  is  the  fiber-cell,  which 
is  a  delicate  spindle  with  its  nucleus  usually  situated  nearer  one 
end  than  the  other.  The  nuclei  of  the  fiber-cells  are  usually 
elongated  and  oval.  These  fiber-cells  are  held  together  by  a 
delicate  connective-tissue  network  which  is  composed  of  both 
white  and  elastic  fibers.  Smooth  muscle  is  very  poorly  supplied 
with  blood-vessels  in  comparison  to  striated  muscle.  The 
blood-vessels  run  along  the  connective-tissue  septa  and  small 
branches  are  distributed  to  the  fiber-cells.  The  lymphatics, 
also,  follow  the  connective-tissue  septa.  The  nerves  which  supply 
the  smooth  muscle  are  of  sympathetic  fibers. 

The  Nervous  Tissues. 

These  tissues  will  be  considered  under  the  chapter  on  the 
Physiology  of  the  Nervous  System. 


CHAPTER  III. 
PHYSIOLOGICAL  CHARACTERISTICS  OF  MUSCLE. 

WHEN  a  muscle  is  acted  upon  by  a  weight  it  extends  quite 
readily,  but  as  soon  as  the  weight  is  removed  the  muscle  re- 
sumes its  normal  shape.  This  illustrates  the  extensibility  and 
elasticity  of  muscular  tissue.  The  muscles  all  over  the  body 
are  in  a  constant  state  of  elastic  tension,  which  causes  them  to 
be  of  greater  value  as  a  support  to  the  body  skeleton.  A  muscle 
which  is  in  a  state  of  elastic  tension  contracts  more  readily  and 
forcibly  than  one  which  is  relaxed. 

Under  ordinary  conditions  a  muscle  receives  the  stimulus 
which  causes  it  to  contract  through  its  motor  nerve  from  the 
central  nervous  system.  If  this  nerve  be  cut,  the  muscle  is 
paralyzed.  However,  it  has  been  demonstrated  that  a  muscle 
which  has  its  nerve  cut  may  still  be  made  to  contract  by  applying 
an  artificial  stimulus,  as  an  electrical  shock.  But  such  a  muscle 
would  still  have  its  nerve  endings  in  the  muscle  undestroyed, 
and  hence,  this  would  not  prove  that  the  muscle  has  independent 
contractility.  Still,  if  the  nerve  is  severed  and  the  nerve  endings 
are  destroyed,  e.  g.,  by  a  drug,  we  find  that  the  muscle  will  still 
respond  to  an  electrical  stimulus.  This  shows  that  muscular 
tissue  has  independent  irritability.  Hence,  striated  muscular 
tissue  possesses  independent  contractility,  by  which  is  meant 
that  its  power  of  shortening  is  due  to  active  processes  developed 
in  its  own  tissue,  and  independent  irritability,  by  which  is 
meant  that  it  may  enter  into  contraction  by  artificial  stimuli 
applied  directly  to  its  own  substance. 

If  we  isolate  a  muscle  and  stimulate  it,  we  get  a  simple  con- 
traction. If  the  end  of  this  muscle  is  attached  to  a  lever  con- 

23 


24  PHYSIOLOGICAL   CHARACTERISTICS    OF    MUSCLE 

nected  with  a  revolving  drum,  we  get  a  simple  muscle  curve 
(Fig.  19).  The  time  required  for  a  simple  contraction  varies 
with  the  muscles  of  different  animals,  and  also  with  different 
muscles  of  the  same  animal.  After  the  muscle  is  stimulated 
(Fig.  19),  an  appreciable  time  elapses,  the  latent  period,  before 
it  contracts,  which  is  about  3-^  second.  Then  the  muscle 
passes  into  the  stage  of  contraction,  during  which  time  the  lever 
rises.  Immediately  it  relaxes  and  elongates  and  the  lever  again 
descends  to  the  base  line.  The  whole  contraction  occupies 
about  T\)  second. 


FIG.  19. — Simple  muscle  curve.     (Halliburton.) 

Those  factors  which  modify  the  character  of  a  simple  muscle 
curve,  are,  .(a)  the  strength  of  the  stimulus,  (b)  the  amount  of  the 
load,  (c)  the  influence  of  fatigue,  (d)  the  effect  of  temperature  and 
(e)  the  effect  of  veratrine. 

(a)  A  stimulus  which  is  just  strong  enough  to  produce  a  con- 
traction is  called  a  minimal  stimulus.  As  the 'strength  of  the 
stimulus  is  increased  the  amount  of  the  contraction,  which  is 
represented  by  the  height  of  the  curve,  is  increased.  This  con- 
tinues until  a  certain  point  is  reached,  the  maximal  stimulus, 


PHYSIOLOGICAL    CHARACTERISTICS    OF    MUSCLE  25 

then  an  increase  in  the  stimulus  produces  no  increase  in  the 
contraction. 

(b)  As  the  weight  of  the  load  is  increased  the  contraction 
becomes  less  until  a  weight  is  reached  which  the  muscle  is 
unable  to  raise.     Also,  the  latent  period  is  longer  with  a  heavy 
than  with  a  light  load. 

(c)  If  we  apply  a  series  of  successive  stimuli  to  a  muscle  we 
notice  that  at  first  the  contractions  improve  with  each  successive 
stimulus  which  is  due  to  the  beneficial  effect  of  contraction. 
Later  the  contractions  get  less  and  less.     As  the  contractions 
get  less,  the  period  of  contraction  becomes  longer,  the  latent 
period  is  increased  and  the  period  of  relaxation  becomes  very 
much  longer.     As  the  period  of  relaxation  becomes  longer,  the 
muscle  fails   to    return   to    its  normal  length  before  a  second 
stimulus  arrives,  so  that  the  original  base  line  is  not  reached  at 
all.     This  condition  is  known  as  contracture. 

(d)  By  varying  the  temperature  of  a  muscle  we  find  that  it 
causes  a  variation  in  the  extent  and  duration  of  its  contractions. 
Thus,  by  beginning  at  o°  C.  and  increasing  the  temperature, 
we  find  that  the  contractions  increase  up  to  5°-9°  C.  and  then 
decrease  up  to   i5°-i8°  C.     After  this  point  is  reached  they 
again  increase  reaching  their  maximum  at  26°-3o°  C.     This 
maximum  is  much  greater  than  the  first  maximum  which  was 
reached  at  5°-9°  C.     As  the  temperature  is  still  increased,  the 
contractions  decrease  rapidly  until  at  about  37°  C.  irritability 
is  entirely  lost.     If  the  temperature  is  increased  to  about  42°  C. 
heat  rigor  makes  its  appearance  due  to  the  coagulation  of  the 
muscle  plasma. 

(e)  Veratrin  is   an   alkaloid  which   exerts   a  peculiar  effect 
upon  the  contraction  of  muscle.     By  injecting  it  into  an  animal 
before  the  muscle  is  removed  the  following  effects  are  noted: 
(i)  The  phase  of  shortening  is  not  altered,  but  the  period  of 
relaxation  is  very  much  prolonged,  and  (2)  there  is  a  secondary 
rise  in  the  curve  of  relaxation. 


20  PHYSIOLOGICAL   CHARACTERISTICS    OF    MUSCLE 

Effect  of  Two  or  More  Successive  Stimuli. — If  a  muscle 
receives  two  successive  stimuli  a  sufficient  length  of  time  apart, 
two  curves  of  contraction  are  produced,  the  second  being  a 
little  higher  than  the  first  (beneficial  effect  of  contraction) .  How- 
ever, if  the  second  stimulus  arrives  before  the  period  of  relaxation 
is  complete,  a  secondary  rise  is  produced  which  is  called 
superposition  or  summation  of  effects.  If  the  two  stimuli  occur 


u 


FIG.  20. 

7,  Two  successive  submaximal  contractions.  //,  A  series  of  contractions  induced 
by  12  induction-shocks  in  a  second.  ///,  Marked  tetanus  induced  by  rapid  shocks. 
(Landois.) 

close  enough  together,  the  result  will  be  one  curve  which  is  greater 
than  either  would  have  produced  separately.  This  is  called 
summation  of  stimuli.  If,  instead  of  just  two  stimuli,  a  number 
of  stimuli  are  applied  very  close  together,  we  get  the  effect  shown 
in  (II).  If  these  stimuli  occur  still  closer  together  the  effect 
shown  in  (III)  is  produced  which  is  called  tetanus.  When 
the  stimuli  occur  so  as  to  allow  partial  relaxation  between  each 
stimulus,  (II)  the  effect  is  called  incomplete  tetanus,  but  when 
no  relaxation  occurs  as  in  (III)  the  effect  is  complete  tetanus. 


CHAPTER  IV. 
SECRETION. 

Secretion  and  Excretion. — Ordinarily  the  product  of  glan- 
dular activity  is  spoken  of  as  a  secretion.  On  the  one  hand, 
glands  may  take  from  the  blood  substances  which  are  preformed 
in  that  fluid,  which  would  accumulate  and  produce  detrimental 
effects  if  not  removed,  and  which  are  discharged  from  the  body. 
On  the  other  hand,  glands  may  form  out  of  materials  furnished 
by  the  blood  substances  which  are  peculiar  to  that  gland's  activ- 
ity, which  have  an  office  to  perform  in  the  economy,  which  do  not 
accumulate  on  removal  of  the  gland,  and  which  are  not  dis- 
charged from  the  body.  The  product  in  the  first  case  is  an 
excretion,  in  the  second  case  a  secretion.  But  when  it  comes 
to  naming  an  exclusively  excretory  or  an  exclusively  secretory 
gland,  the  task  is  found  to  be  practically  impossible.  Probably 
the  most  typical  excretion  of  the  body  is  the  urine,  yet  there 
are  in  the  urine  substances,  like  hippuric  acid,  etc.,  which  are 
undoubtedly  formed  by  the  kidney,  and  which  do  not  preexist  in 
the  blood.  The  succus  entericus,  e.  g.,  would  seem  as  typical  a 
secretion  as  it  is  possible  to  find,  but  not  infrequently  it  contains 
urea  when  the  activity  of  the  kidney  is  impaired,  to  say  nothing, 
under  normal  conditions,  of  the  water  and  salts  which  are  taken 
as  such  from  the  blood.  The  liver  is  notable  in  its  secreto- 
excrementitious  action.  While  the  desirability  of  thus  separat- 
ing the  glands  into  secretory  and  excretory  and  their  products 
into  secretions  and  excretions  is  granted,  the  impossibility  of 
such  a  division  is  apparent. 

It  is  possible  in  most  cases  to  apply  the  distinction  to  the 
separate  constituents  of  the  product  of  a  particular  gland,  but 

27 


28  SECRETION 

not  to  the  product  as  a  whole.  In  view  of  these  facts,  attention 
will  be  given  in  this  chapter  to  several  glands  which  manifestly 
produce  excretions  as  well  as  secretions.  The  action  of  the 
kidney  and  sweat  glands  is  so  predominantly  excretory  that  they 
are  treated  separately.  In  what  follows  the  term  " secretion" 
cannot  always  be  taken  as  meaning  a  true  secretion,  for  it  is 
customary  and  convenient  to  speak  of  the  "  secretion  of  urine," 
for  example. 

Glands. — If  we  conceive  of  a  single  layer  of  secreting  epi- 
thelial cells  supported  by  a  thin  basement  membrane,  and  then 
this  structure  invaginated  or  folded  in  upon  itself,  so  that  the 
two  layers  of  epithelium  face  each  other  with  a  greater  or  less 
interval  between  them,  with  the  basement  membrane  constitut- 
ing the  external  support  for  both,  we  will  have  in  mind  the 
essential  structure  of  a  gland  proper.  The  invaginated  cells  are 
the  gland  cells,  and  the  interval  between  the  two  layers  of  cells 
is  the  lumen.  Whether  the  invaginated  structure  sends  off 
from  itself  secondary  or  tertiary  folds  similar  to  the  original,  or 
whether  the  lumen  of  any  of  these  folds  is  in  the  shape  of  a  simple 
tube  or  sac,  or  both,  is  immaterial.  They  may  all  be  considered 
as  identical  in  nature  with  the  original  invaginat'.on  and  only 
modifications  of  its  architecture. 

However,  these  modifications  are  more  or  less  distinguished  by 
names.  Those  which  become  complex  by  numerous  branch- 
ings of  the  involuted  tube  are  usually  termed  compound,  as 
opposed  to  a  single  simple  fold;  glands  are  further  classified,  as 
tubular,  racemose,  or  tubulo-racemose,  according  as  the 
termination  of  the  lumen  has  the  shape  of  a  tube,  or  sac,  or  both. 
Thus  a  simple  or  a  compound  gland  may  belong  to  any  one  of 
the  three  last-named  varieties.  The  crypts  of  Lieberkuhn  are 
simple  tubular  glands.  The  glands  of  Brunner  are  usually 
described  as  compound  tubulo-racemose  structures. 

In  a  compound  gland  that  portion  which  communicates  with 
the  surface  is  called  the  duct  and  is  supposed  not  to  be  con- 


GLAND   SECRETION  29 

cerned  in  actual  secretion,  but  simply  in  carrying  the  product 
away  from  the  secreting  terminal  ramifications  of  the  subdivi- 
sions of  the  involution — which  terminations  are  called  acini  or 
alveoli.  It  follows,  of  course,  that  a  collection  of  acini  may  dis- 
charge their  secretion  into  the  main  duct  by  a  smaller  duct — 
that  is,  that  the  gland  may  have  various  subdivisions  of  the  duct 
proper. 

Furthermore  secretions  are  classified  as  external  when  they 
are  discharged  upon  a  surface  communicating  with  the  external 
air,  such  as  the  alimentary  canal,  or  skin,  and  internal  when 
they  are  discharged  upon  surfaces  not  in  communication  with 
the  exterior,  such  as  blood-vessels.  Both  external  and  internal 
secretions  are  liquid  or  semi-liquid  in  character,  for  they  must 
contain  water  as  a  vehicle  for  the  salts  and  organic  substances 
which  are  present  in  all  of  them  and  which,  in  fact,  distinguish 
them  from  one  another. 

Glands  in  general  have  been  divided  into  serous  and  mucous 
by  Heidenhain,  according  as  the  secreted  fluid  is  watery  and 
thin,  or  viscid  and  stringy  from  the  presence  of  mucin.  This 
division  is  further  warranted  by  histologic  differences  in  the  cells 
concerned  in  each  kind  of  secretion.  The  cells  in  a  serous 
gland  are  small  and  finely  granular,  and  are  in  close  apposition 
to  each  other.  Those  of  mucous  glands  are  larger,  almost 
square  and  are  definitely  separated.  Many  glands  contain  both 
kinds  of  cells,  but  since  their  secretion  contains  mucin,  such 
glands  are  usually  spoken  of  as  belonging  to  the  mucous  variety. 
It  will  be  seen  that  the  salivary  glands  illustrate  these  varieties. 

Gland  Secretion. — Underneath  the  basement  membrane  of  a 
gland  (that  is,  on  the  side  opposite  the  epithelial  cells)  ramifies 
an  abundant  network  of  blood  and  lymph  capillaries.  This  an- 
atomical arrangement  favors  osmotic  transudation  from  the  ves- 
sels, especially  since  the  pressure  in  the  vessels  is  normally 
greater  than  in  the  acini  and  ducts  of  the  gland.  Numerous 
experiments,  however,  prove  the  inadequacy  of  simple  osmosis 


30  SECRETION 

to  explain  all  the  processes  of  glandular  secretion,  especially 
those  connected  with  the  presence  of  organic  constituents; 
while  the  undoubted  presence  of  secretory  nerves  (besides  the 
vaso-motor  nerves  to  the  vessels)  would  seem  to  give  a  priori 
evidence  that  the  glandular  epithelium  takes  some  active  part 
in  the  formation  of  the  secretion.  Such  an  office  is  granted  to 
these  cells,  but  whether  it  is  of  chemical,  or  a  physical,  or  a 
"vital"  character  is  not  evident. 

The  physiology  of  the  salivary  glands,  the  gastric  and  intestinal 
glands,  the  pancreas  and  liver  is  taken  up  under  the  chapter  on 
Digestion  in  which  they  are  vitally  concerned. 

Sebaceous  Glands. 

The  sebaceous  glands  (see  Hair-follicles)  are  chiefly  associ- 
ated with  hair-follicles  and,  existing  wherever  hair  is  to  be  found, 
cover  well-nigh  the  whole  cutaneous  surface.  They  are  of  the 
simple  or  compound  tubular  type,  and  discharge  their  secretion 
into  the  hair-follicle  near  its  outer  extremity.  The  alveoli  are 
lined  by  several  layers  of  cuboidal  epithelial  cells.  The  cells 
of  the  layer  nearest  the  lumen  contain  fatty  matter,  and  are 
thought  to  form  the  secretion  by  breaking  down  and  being  thrown 
off  themselves.  Their  place  is  taken  by  cells  from  the  deeper 
layers,  which  undergo  similar  changes  and  disintegrate. 

Composition  and  Properties  of  Sebum. — Chemically  sebum 
is  largely  made  up  of  fatty  matters.  It  also  contains  choles- 
terin,  which  is  in  combination  with  a  fatty  acid.  It  forms  a  thin 
coating  over  the  cutaneous  surface,  accounting  for  the  normal 
oiliness  of  the  skin.  It  also  contributes  to  the  characteristic 
softness  of  the  hairs,  and  prevents  their  breaking  off  from  brittle- 
ness.  Its  presence  over  the  body  surface  may  have  some  influ- 
ence in  regulating  the  loss  of  heat  by  evaporation. 

Cerumen,  smegma  and  the  secretion  from  the  Nabothian 
glands  are  only  modified  forms  of  sebum,  and  the  structures 
producing  these  secretions  belong  to  the  class  of  sebaceous  glands. 


MAMMARY   GLANDS  31 

Mammary  Glands. 

Structure. — The  mammary  glands  are  two  in  number  in 
the  human  being,  and  are  loosely  attached  to  the  great  pectoral 
muscles.  They  are  rudimentary  in  both  sexes  until  puberty, 
and  in  men  throughout  life.  At  puberty  the  gland  in  the  female 
enlarges  markedly,  but  is  never  fully  developed  before  preg- 
nancy. At  this  time  the  gland  vesicles  make  their  appearance, 
and  the  rudimentary  ducts  come  to  be  more  and  more  ramified. 
These  ramifications  do  not  reach  their  full  development,  how- 
ever, until  lactation  begins.  The  skin  covering  the  areola  of 
the  nipple  is  dark,  especially  during  pregnancy,  and  much  thinner 
than  over  other  parts.  The  dark  color  is  due  to  a  deposit  of 
pigment. 

The  mammary  gland  belongs  to  the  compound  tubulo-race- 
mose  type,  and  consists  of  fifteen  or  twenty  lobes  bound  together 
by  areolar  connective  tissue.  Each  lobe  is  made  up  of  a  num- 
ber of  lobules,  containing  the  alveoli  or  secreting  portions. 
The  secretion  from  all  the  alveoli  and  lobules  of  a  lobe  converges 
to  a  single  duct,  which  discharges  its  contents  upon  the  surface  of 
the  nipple  without  anastomosis  with  any  duct.  There  are, 
therefore,  some  fifteen  or  twenty  ducts  thus  opening  upon  the 
surface.  Each  of  them  has  a  dilatation  beneath  the  nipple,  and 
it  is  in  these  sinuses  largely  that  the  milk  accumulates  during 
lactation.  When  lactation  has  ceased  the  ducts  retract,  the  sin- 
uses disappear,  the  alveoli  undergo  retrograde  changes,  and  the 
whole  gland  is  inclined  to  become  flabby  and  pendulous.  It 
does  not  regain  after  pregnancy  the  firmness  which  character- 
ized it  before. 

Secretion  of  Milk.— After  parturition  the  first  discharge 
from  the  gland  is  colostrum,  a  liquid  resembling  milk  in  some 
respects.  In  two  or  three  days  the  true  milk  appears.  Besides 
water  and  salts,  all  the  constituents  of  milk  are  formed  by  the 
cells  of  the  mammary  gland.  During  the  period  of  gestation  the 


32  SECRETION 

cells  lining  the  alveoli  are  flat  and  have  only  a  single  nucleus. 
When  they  begin  to  secrete  they  increase  in  height,  the  nuclei 
divide  and  that  portion  of  the  cell  toward  the  lumen  undergoes 
fatty  degeneration.  This  fatty  material  is  extruded  into  the 
lumen  and  apparently  constitutes  a  part  of  the  secretion.  The 
liquid  constituents  taken  out  of  the  blood  probably  hold  the  pro- 
teid  and  carbohydrate  portions  in  solution,  while  the  fatty  par- 
ticles constitute  the  fat  of  the  milk.  Thus  secreted,  the  liquid 
accumulates  in  the  ducts  and  sinuses  until  removed  by  the  infant 
or  otherwise.  The  fact  that  the  secretion  of  milk  in  woman  is  in- 
fluenced by  emotions  of  fear,  grief,  etc.,  is  strong  evidence  of  a 
nervous  control  of  the  procedure,  but  proof  of  secretory  fibers  to 
the  cells  has  not  been  established. 

The  quantity  of  food  required  by  the  mother  during  the  time 
the  child  is  nursed  is  increased,  but  no  particular  kind  of  food 
seems  to  be  especially  required.  The  larger  demand  for  liquids 
is  marked,  however,  and  when  the  quantity  of  milk  is  increased 
by  a  large  ingestion  of  liquids,  the  solids  in  the  secretion  are  not 
relatively  diminished. 

Composition  and  Properties  of  Milk.— Human  milk  has  a 
specific  gravity  of  about  1030,  and  is  not  so  white  or  so  opaque 
as  cow's  milk.  Besides  water,  its  chief  constituents  are  fats,  lec- 
ithin, cholesterin,  casein  and  lactose,  of  which  the  two  last  named 
are  the  most  important.  Casein  is  the  main  proteid  constituent. 
Lactose  is  very  abundant,  and  is  responsible  for  the  sweet  taste 
and  for  a  large  part  of  the  nutritive  value  of  the  fluid. 

Thyroid  Gland. 

The  thyroid  gland  consists  of  two  glandular  masses  united  by 
an  isthmus  of  the  same  structure.  It  lies  in  front  of  the  trachea 
at  the  lower  end  of  the  larynx.  It  consists  of  a  large  number 
of  vesicles  bound  together  by  connective  tissue.  Each  vesicle 
is  lined  by  cuboidal  epithelial  cells,  which  secrete  a  semi-gelatin- 
ous substance,  colloid. 


THYROID  AND  ADRENAL    GLANDS  33 

It  has  long  been  known  that  the  removal  of  the  whole  thyroid 
gland,  including  the  parathyroid,  occasioned  marked  interfer- 
ence with  nutrition  and  other  changes,  the  chief  of  which  are 
disturbances  of  muscular  coordination,  possibly  convulsions, 
emaciation,  apathy,  and  subsequent  death.  There  is  no  duct 
connected  with  the  gland,  and  the  secretion  is  therefore  an  in- 
ternal one.  Very  little  is  known  of  it  except  that  it  is  necessary 
to  the  maintenance  of  life.  If  a  very  little  of  the  gland  be  left, 
or  if,  after  its  complete  removal,  a  small  bit  of  it  be  transplanted 
in  some  other  part  of  the  body,  or  if  the  animal  be  fed  on  the 
thyroid  extract  or  the  fresh  gland,  the  characteristic  symptoms 
do  not  ensue. 

The  muscular  disturbances  direct  the  attention  to  the  central 
nervous  system  when  an  attempt  is  made  to  explain  the  occur- 
rences and  it  is  not  improbable  that  the  effect  of  the  thyroid 
secretion  is  in  some  way  exerted  upon  or  through  the  central 
system.  It  seems  generally  agreed  that  the  thyroid  does  dis- 
charge a  secretion  into  the  blood  and  that  it  is  the  withdrawal 
of  some  part  of  that  secretion  from  the  circulation  which  is  re- 
sponsible for  the  remarkable  train  of  symptoms  sequent  upon  its 
removal.  This  essential  constituent  is  regarded  by  some  as 
being  an  agent  which  destroys  certain  toxic  principles  in  the 
blood,  by  others  as  being  requisite  to  the  metabolic  functions  in 
the  body  without  destroying  anything.  Baumann  has  isolated 
from  the  gland  substance  a  material  containing  a  large  propor- 
tion of  iodine,  to  which  he  gives  the  name  iodothyrin,  and  it  is 
very  probable  that  this  is  one,  at  least  of  the  beneficial  substances 
in  the  thyroid  secretion. 

Adrenal  Glands. 

The  adrenal  gland  or  suprarenal  capsules,  rest  ng  upon  the 
upper  ends  of  the  kidneys,  are  ductless  glands  whose  removal  is 
followed  by  weakness,  impaired  nutrition  and  disturbances  in  the 
circulation.  Death  usually  supervenes  in  two  to  four  days. 

3 


34  SECRETION 

These  bodies  must  produce  an  internal  secretion  which  is  re- 
moved by  way  of  the  adrenal  veins.  It  may  destroy  toxic  sub- 
stances in  the  blood.  A  solution  injected  into  the  circulation 
certainly  affects  the  middle  wall  of  the  vessels,  causing  contrac- 
tion, and  a  heightened  pressure.  The  heart  is  also  notably 
inhibited.  It  is  not  thought  that  the  effect  on  the  vessels  is 
brought  about  through  the  vaso-motor  nerves,  but  by  direct 
excitation  of  the  muscular  substance.  Little  in  fact  is  known 
about  the  secretion,  except  that  it  is  necessary  to  life.  Abel  has 
isolated  an  alkaloid,  epinephrine,  which  is  claimed  to  be  the 
active  principle.  These  glands  are  the  seat  of  lesions  in  Addison's 
disease,  and  many  cases  of  this  malady  are  at  least  favorably 
influenced  by  the  use  of  adrenal  extract. 

Pituitary  Body. 

The  pituitary  body  lying  in  the  sella  turcica  on  the  superior 
surface  of  the  sphenoid  bone,  also  produces  an  internal  secretion 
of  physiological  value.  Its  removal  is  regarded  as  causing  death. 
Howell  has  shown  that  injection  of  extract  from  the  posterior 
division  occasions  a  rise  of  temperature  and  slowing  of  the  heart. 
Its  situation  makes  satisfactory  experiments  very  difficult. 

Testis  and  Ovary. 

The  testes  and  ovaries,  though  not  probably  true  glands, 
also  may  produce  an  internal  secretion  of  obscure  physiological 
value.  It  is  not  essential  to  life.  Injections  of  extracts  from 
these  bodies  are  claimed  to  have  a  remarkable  stimulating  effect 
upon  the  nervous  and  muscular  systems.  In  mental  and  physical 
disturbances  occasionally  following  removal  of  the  ovaries,  gyne- 
cologists often  find  administration  of  the  ovarian  extract  to  be 
beneficial. 


CHAPTER  V. 
THE  BLOOD. 

General  Characteristics. — The  blood  is  a  red,  opaque  and 
viscid  fluid  having  a  characteristic  stale  odor  and  a  salty  taste. 
The  blood  is  heavier  than  water,  having  a  specific  gravity  in 
the  adult  male  of  1.041  to  1.067,  tne  average  being  about  1.055. 

The  reaction  of  the  blood  is  neutral.  The  nature  of  the  diet, 
either  meat  or  vegetable,  causes  this  neutrality  to  turn  to  either 
an  acid  or  an  alkaline  reaction. 

The  blood  temperature  is  that  of  the  body.  In  the  periphery 
it  is  about  99°  F.;  in  deeper  vessels  it  varies  from  100°  F.  to  107° 
F. ;  and  in  the  hepatic  veins  it  is  about  107°  F. 

The  Function  of  the  Blood.— The  most  important  physio- 
logical functions  of  the  blood  are:  (i)  It  carries  to  the  tissues 
food-stuffs  after  they  have  been  digested,  (2)  it  transports  to  the 
tissues  oxygen  which  it  has  absorbed  from  the  air  in  the  lungs,  (3) 
it  carries  off  from  the  tissues  the  waste  products  of  metabolism, 
(4)  it  transmits  the  internal  secretions  of  glands  to  the  differ- 
ent parts  of  the  body  and  (5)  it  aids  in  equalizing  the  body 
temperature. 

Quantity  and  Distribution  of  the  Blood. — The  quantity  of 
the  blood  in  the  body  is  estimated  at  about  7.5  per  cent,  of  body 
weight.  A  man  weighing  150  pounds  has  a  fraction  over  eleven 
pounds  of  blood,  which  is  about  one-twelfth  of  the  body  weight. 

The  distribution  is  generally  given  as,  one-fourth  in  the  heart, 
large  arteries,  lungs,  and  veins;  one-fourth  in  the  liver;  one- 
fourth  in  the  muscles  attached  to  the  skeleton;  and  the  other 
one-fourth  variously  distributed  to  the  other  organs  of  the  body. 

35 


36  THE   BLOOD 

Composition  of  Blood. 

The  blood  is  composed  of  a  fluid  part,  the  plasma,  in  which 
float  a  great  mass  of  small  bodies,  the  blood  corpuscles.  The 
plasma  may  be  denned  as  the  blood  minus  the  corpuscles.  These 
are  of  three  varieties:  (i)  The  red  corpuscles,  or  erythrocytes, 
(2)  the  white  corpuscles,  or  leucocytes  and  (3)  the  blood  platelets, 
or  thrombocytes.  The  plasma  is  a  thin  slightly  yellowish  fluid 
with  a  specific  gravity  of  1.026  to  1.029.  Hence,  the  bright  red 
color  of  the  blood  is  due  to  the  red  corpuscles  which  are  held  in 
suspension  in  the  plasma.  The  proportion  of  plasma  to  cor- 
puscles is  about  two  to  one  (Ho well). 

Plasma. 

Chemically,  plasma  is  composed  of  water  and  about  10  per 
cent,  of  solids,  together  with  oxygen,  carbon  dioxide  and  nitrogen. 
A  thousand  parts  of  plasma  contain:  (Halliburton.) 

Water 902  . 90 

Solids 97  . 10 

Proteins:  i.  Yield  of  fibrin 4.05 

2.  Other  proteins 78 .84 

Extractives  (including  fat) 5-66 

Inorganic  salts 8.55 

The  most  important  solids  are  the  proteins,  the  chief  of  which 
are:  (i)  Fibrinogen,  (2)  serum  globulin,  and  (3)  serum  albumin. 
Fibrinogen  belongs  to  the  globulin  class  of  proteins,  but  differs 
from  serum  globulin  and  may  be  separated  from  it.  Fibrinogen 
is  the  least  abundant  of  the  proteins.  Serum  globulin  and  serum 
albumin  form  the  chief  proteins  of  the  plasma.  They  may  be 
separated  by  the  use  of  neutral  salts. 

The  extractives  are  substances  other  than  proteins  which 
may  be  extracted  from  the  dried  residue  by  the  use  of  water, 
alcohol,  or  ether.  The  principal  extractives  are  fats,  sugar, 
lecithin,  cholesterin,  lactic  acid  and  urea. 


RED   BLOOD    CORPUSCLES  37 

The  most  abundant  salt  of  the  plasma  is  sodium  chloride. 
It  forms  from  60  to  90  per  cent,  of  the  total  mineral  matter  of 
plasma.  Potassium  chloride  is  present  in  much  smaller  amount. 
Other  salts  are  the  carbonates,  sulphates  and  phosphates. 

Corpuscles. 

Suspended  in  the  plasma  of  the  blood  we  have  a  cellular  formed 
element  moving  and  functionating.  This  element  is  the  corpus- 
cular element  and  is  composed  of  (a)  the  red  blood  corpuscles,  (b) 
the  white  blood  corpuscles,  (c)  the  blood  platelets. 

(a)  Red  Blood  Corpuscles  or  Erythrocytes. 

General  Description. — The  red  blood  corpuscles  are  circular, 
bi-concave  discs  with  rounded  edges.  They  are  from  7  to  8 
micra  in  diameter  and  2  micra  in  thickness,  so  can  only  be  seen 
with  the  aid  of  the  microscope.  When  looked  at  singly  they 
appear  to  have  a  yellowish-green  color,  collectively  they  are 
red. 

Number. — In  males  there  are  about  5,000,000  red  cells  to  a 
cubic  millimeter;  in  females  about  4,500,000.  The  proportion 
of  reds  to  whites  is  one  white  to  every  500  red. 

Origin  and  Destruction. — The  red  corpuscles  are  continu- 
ally being  destroyed  in  the  body.  It  appears  that  this  destruction 
occurs  principally  in  the  liver.  As  the  red  cells  are  thus  de- 
stroyed it  is  natural  to  look  for  a  place  of  manufacture.  In  the 
embryo  we  find  that  this  generation  takes  place  in  the  liver  and 
in  the  spleen;  in  the  adult  it  seems  that  the  manufacture  takes 
place  only  in  the  red  marrow  of  the  bones. 

The  red  corpuscles  are  formed  from  colored,  nucleated  cells 
called  hemoblasts. 

Constituents  of  Red  Blood  Corpuscles.— The  red  blood  cor- 
puscles are  made  up  of  65  per  cent,  water  and  35  per  cent,  solids. 
The  principal  solid  constituents  are  (a)  hemoglobin  (oxyhemo- 


3»  THE    BLOOD 

globin)  87-95  Per  cent.;  (b)  stroma,  composed  of  fat,  lecithin, 
and  cholesterin;  (c)  and  salts,  principally  potassium  chloride, 
and  potassium  phosphate. 

Hemoglobin. — Hemoglobin  is  the  coloring  matter  of  the 
red  cells,  and  is  composed  of  (i)  hematin,  a  pigment  containing 
iron;  and  (2)  globin,  a  proteid.  Hemoglobin  is  of  great  physio- 
logical importance,  because  of  its  ability  to  unite  with  oxygen 
and  thus  form  oxyhemoglobin.  By  it  the  blood  carries  its 


FIG.  21. 

A,  human  colored  blood  corpuscles — i,  on  the  flat;  2,  on  edge;  3,  rouleau  of  colored 
corpuscles.  B,  amphibian  colored  blood  corpuscles — i,  on  the  flat;  2,  on  edge. 
C,  ideal  transverse  section  of  a  human  colored  blood-corpuscle  magnified  5,000 
times  linear — a,  b,  diameter;  c,  d,  thickness.  (Landois.) 


oxygen  from  the  lungs  to  the  tissues.  It  also  unites  to  some 
extent  with  carbon  dioxide  and  it  is  thus  that  carbon  dioxide  is 
brought  from  the  tissues.  We  find  oxyhemoglobin  chiefly  in  the 
arterial  blood,  while  in  venous  blood  we  find  both  hemoglobin 
and  oxyhemoglobin.  In  asphyxiated  blood  we  find  only 
hemoglobin. 

The  stroma  is  the  colorless  framework  of  the  corpuscles  after 


BLOOD   PLATELETS.  39 

the    coloring    matter    is    dissolved    out.     The    hemoglobin    is 
ensnared  in  the  stroma. 

(b)  White  Blood  Corpuscles  or  Leucocytes. 

General  Description. — The  white  blood  corpuscles  or  leu- 
cocytes are  large,  colorless,  nucleated  cells  with  no  general 
form,  but  which  are  capable  of  changing  their  form  by  ameboid 
movement. 

Number. — The  number  of  leucocytes  varies  from  seven  to  ten 
thousand  per  cubic  millimeter. 

Function. — The  white  corpuscles  are  not  under  the  control  of 
the  central  nervous  system,  but  are  controlled  by  some  chemo- 
taxic  force.  They  are  able  to  go  and  come  by  ameboid  move- 
ment through  the  stromata  of  capillary  walls  and  wander  here 
and  there  in  the  tissues.  It  is  this  that  gives  them  their  name 
of  wandering  cells. 

White  blood  corpuscles  are  of  importance  from  a  physiological 
standpoint,  because  of  this  ability  to  wander.  They  can  trans- 
fer undissolved  substances  from  one  part  of  the  body  to  another 
and  can  destroy  and  remove  foreign  substances  and  hurtful 
microorganisms. 

The  power  they  have  of  ingesting  foreign  substances  is  called 
phagocytosis.  They  will  migrate  in  large  numbers  and  sur- 
round a  foreign  object  and  endeavor  to  remove  it  from  the  tissue. 
They  have  the  power  of  liquefying  tissue  and  it  is  this  lique- 
fied tissue  mixed  with  the  dead  bodies  of  white  corpuscles  that 
is  known  as  pus. 

(3)  Blood  Platelets. 

These  are  colorless  discs  about  one-third  to  one-fourth  the 
size  of  red  blood  corpuscles.  Some  claim  for  them  the  full 
value  of  blood  cells,  while  some  claim  they  are  the  nuclear  re- 
mains of  destroyed  leucocytes.  They  are  about  635,000  to  one 


40  THE   BLOOD 

cubic  millimeter  of  blood.  As  to  their  function  little  is  known. 
Some  claim  they  play  an  important  part  in  the  coagulation  of  the 
blood.  Nothing  definite  is  known  of  their  origin. 

The  Coagulation  of  the  Blood. 

When  blood  is  allowed  to  stand  after  being  shed  it  rapidly 
becomes  more  viscous  and  later  sets  into  a  firm  jelly.  Later, 
as  the  fibrin  contracts,  a  clear  straw  colored  fluid,  the  serum,  is 
set  free.  The  formation  of  fibrin  is  the  essential  factor  in 
coagulation.  It  is  contained  in  the  plasma  in  the  form  of 
fibrinogen. 

The  relation  of  plasma,  serum  and  clot  is  shown  by  the  fol- 
lowing table: 

(  Serum 
Plasma      <   _.,    .        ^ 

Corpuscles  •  J 


CHAPTER  VI. 
THE  CIRCULATION  OF  THE  BLOOD. 

General. — We  have  seen  that  the  composition  of  the  blood  fits 
it  for  its  function  of  carrying  food  stuffs  to  the  tissues  and  remov- 
ing the  products  of  combustion;  but,  for  the  blood  to  exercise 
these  offices,  it  is  necessary  that  it  be  in  communication  with  the 
outside  world  and  the  tissues.  The  movement  it  makes  through 
its  network  of  vessels  in  order  to  carry  products  from  the  exterior 
to  the  interior  and  from  the  interior  to  the  exterior  is  what  is 
meant  by  circulation. 

Pulmonary  and  Systemic  Circulation. — Two  systems  of 
circulation  are  generally  distinguished.  The  first  is  the  pul- 
monary, and  is  the  circulation  of  the  blood  through  the  lungs  in 
order  to  get  rid  of  carbon  dioxide  and  to  get  a  fresh  supply  of 
oxygen  by  aeration.  The  second  is  the  systemic  and  is  the 
circulation  through  the  great  masses  of  body  tissue  in  order,  by 
means  of  the  lymph,  to  supply  the  tissues  with  different  solid, 
liquid,  and  gaseous  nutritive  material  and  take  from  the  tissues 
the  products  no  longer  needed  but  which  must  be  eliminated. 
These  systems  are  also  called  respectively  the  lesser  and  greater 
circulation. 

Discovery. — The  circulation  of  the  blood  was  an  unknown 
fact  up  to  1628  when  the  discovery  of  its  movements  was  made 
and  proved  by  Sir  William  Harvey,  an  English  physician  promi- 
nent in  his  time  and  now  famous  for  this  discovery. 

The  Circulatory  Apparatus. — The  blood  circulates  through 
a  series  of  closed  tubes  known  as  blood-vessels,  which  divide 
up,  ramify,  and  go  to  all  parts  of  the  body.  These  vary  from 


42  THE    CIRCULATION   OF    THE    BLOOD 

large,  macroscopical  vessels  to  tiny,  little,  hair-like  tubes  that 
cannot  be  seen  with  the  naked  eye. 

The  central  organ  of  the  circulatory  system  is  the  heart. 
From  this  lead  off  the  arteries,  these  in  turn  connect  with  the 
capillaries,  and  these  with  the  veins,  which  lead  back  to  the 
heart. 

I.  THE  HEART. 

The  heart  is  a  hollow,  muscular  organ  divided  by  a  muscular 
septum  into  two  distinct  compartments  designated  for  conveni- 
ence, the  right  and  left  heart.  The  right  side,  and  similarly  the 
left,  is  divided  by  a  muscular  septum  into  two  chambers,  the 
upper  called  the  auricle  and  the  lower  the  ventricle.  There  is 
an  opening  between  the  right  auricle  and  the  right  ventricle  and 
one  between  the  left  auricle  and  the  left  ventricle  and  each  open- 
ing is  guarded  and  can  be  closed  by  a  thin  membranous  flap 
called  a  valve. 

Situation. — The  heart  is  located  in  the  thoracic  cavity  be- 
hind the  sternum.  It  is  placed  in  a  diagonal  position  and  its 
base  is  in  the  middle  line  and  looks  backward,  upward,  and  to 
the  right.  Its  apex  is  three  inches  to  the  left  of  the  median  line, 
a  half  inch  internal  to  the  nipple,  and  in  the  fifth  intercostal  space. 

Covering  and  Lining.— A  serous  sac,  called  the  pericardium, 
covers  the  heart.  It  hugs  the  muscle  of  the  heart  closely,  com- 
pletely enveloping  the  organ,  then  turns  back  on  itself  leaving  a 
space  between  the  outer  layer  and  the  layer  next  to  the  muscle. 
In  this  space  is  a  fluid  which  acts  as  a  lubricant. 

The  heart  is  lined  by  a  membrane  called  the  endocardium, 
which  is  composed  of  epithelial  tissue. 

Structure.— The  muscle  of  the  heart  is  striated,  but  contrary 
to  the  usual  rule,  is  involuntary  in  its  action.  The  muscle  fibers 
run  circularly,  obliquely,  and  some  in  the  form  of  the  figure  eight, 
thus  giving  the  power  to  contract  and  pump  the  blood  on  into 
the  circulation. 


THE   HEART  43 

Contraction. — The  physiological  contraction  of  the  cardiac 
muscle  is  called  systole,  the  relaxation  is  called  diastole.  The 
contraction  of  the  heart  starts  at  the  mouth  of  the  veins  and, 
with  a  uniform  rhythm  glides  along  through  the  auricles  and 
along  to  the  ventricles,  each  part  relaxing  as  the  rhythmic  con- 
traction passes  on.  The  whole  time  of  contraction,  from  one 


FIG.  22. — Scheme  of  cardiac  cycle. 

The  inner  circle  shows  the  events  which  occur  within  the  heart ;  the  outer  the  rela- 
tion of  the  sounds  and  pauses  to  these  events.  (Kirkes  after  Sharpey  and  Gairdner.) 

beginning  in  the  veins  to  another  beginning,  is  called  the  cardiac 
cycle.  It  lasts  about  .86  second. 

The  cycle  may  be  divided  thus;  the  auricles  contract  (systole) 
and  ventricles  are  relaxed  (diastole)  which  occupies  .16  second; 
the  ventricles  contract  (systole)  and  the  auricles  are  relaxed 
(diastole)  and  this  occupies  .3  second;  both  auricles  and  ventricles 
then  rest  and  this  occupies  .4  second. 

Number  of  Beats.  —  In  an  adult  the  heart  beats  on  an  average 
of  72  times  per  minute,  in  children  it  is  higher.  The  frequency 
of  beat  is  influenced  by  age,  sex,  disease,  drugs,  physical  causes 
and  digestion. 


44  THE    CIRCULATION    OF    THE    BLOOD 

Valves  and  Openings. 

Right  Auricle. — Leading  off  from  the  right  auricle  anteriorly 
and  superiorly  is  a  sinus  that  bears  the  name  of  the  auricular 
appendix.  It  is  a  little  hollow  pouch  capable  of  distention  with 
blood. 

Opening  into  the  right  auricle  we  find  the  coronary  veins,  the 
two  venae  cavae,  and  the  auriculo-ventricular  opening.  Guard- 
ing these  openings  are  valves  to  prevent  the  backward  flow  of 
the  blood  current. 

Right  Ventricle. — Opening  into  the  right  ventricle  are  the 
pulmonary  artery  and  the  right  auricle. 

The  tricuspid  valve  guards  the  auriculo-ventricular  opening. 
It  is  composed  of  three  triangular  shaped  membranes  attached 
to  the  base  of  the  circumference  of  the  opening  and  the  apices 
of  the  triangles  coming  together  when  closed. 

The  semi-lunar  valves  guard  the  pulmonary  opening.  They 
are  three  entirely  separate  segments  of  semi-lunar  shape  and  are 
attached  by  their  long  curved  margins  to  the  circumference  of 
the  artery  just  where  it  springs  from  the  muscular  substance  of 
the  ventricles. 

Left  Auricle. — Like  the  right  auricle,  this  cavity  has  a  small 
sinus  leading  off  from  it  anteriorly  and  superiorly — the  auricular 
appendix.  The  openings  into  the  left  auricle  are  the  four  pul- 
monary veins  and  the  left  ventricle. 

Left  Ventricle. — This  ventricle  has  the  thickest  walls  and 
does  the  most  work  of  any  of  the  chambers  of  the  heart,  because 
it  forces  the  fresh  arterial  blood  out  into  the  aorta  and  thence 
through  the  entire  systemic  circulation. 

The  aorta  and  the  left  auricle  open  into  this  ventricle.  The 
aortic  semi-lunar  valves  guard  the  aortic  opening.  They  are 
three  distinct  semi-lunar  shaped  membranes  to  close  the  aortic 
opening  at  the  end  of  the  systole.  The  mitral  or  bicuspid 
valve  closes  the  left  auriculo-ventricular  opening.  It  is  some- 


VALVES  AND    OPENINGS  45 

what  like  the  tricuspid  except  that  it  has  only  two  flaps  instead 
of  three. 

Functions  of  Valves. — The  valves  are  arranged  at  the  open- 
ings of  the  different  chambers  of  the  heart  so  the  blood 
will  be  forced  in  a  constant  direction.  When  the  auricles 
are  at  systole  the  auriculo-ventricular  valves  are  open  thus 
letting  the  flow  of  blood  go  from  auricles  to  ventricles;  but 
as  soon  as  auricular  diastole  and  ventricular  systole  begin 
these  valves  shut  and  the  blood  is  kept  from  flowing  backward 
into  the  auricles.  Then  the  semi-lunar  valves  are  open  and  the 
blood  is  forced  into  the  aorta  and  pulmonary  artery.  When 
ventricular  diastole  begins  these  semi-lunar  valves  closed  and 
thus  blood  is  prevented  from  running  back  into  the  heart  from 
the  arteries. 

Work  of  the  Heart. — The  work  done  by  the  heart  is  equal  to 
the  weight  of  a  column  of  blood  multiplied  by  the  height  or  dis- 
tance to  which  this  column  is  carried  by  the  heart  force.  The 
column  of  blood  is  that  amount  that  is  sent  by  a  single  contrac- 
tion of  the  heart  and  the  height  to  which  it  is  carried  is  equal  to 
the  pressure  in  the  aorta  and  pulmonary  arteries. 

The  amount  of  blood  thrown  into  the  aorta  at  each  con- 
traction of  the  ventricles  weighs  about  87  grams  (about  3  oz.) 
and  the  height  to  which  it  is  forced  is  about  1.5  meters  or  5  feet 
in  man. 

In  estimating  the  work  of  a  machine  the  English  express  the 
result  in  foot  pounds.  The  French  in  grammetres.  A  foot 
pound  is -the  energy  expended  in  raising  a  unit  weight  (i  Ib.) 
through  a  unit  distance  (i  ft.).  A  grammetre  is  the  force  ex- 
pended in  raising  one  gram  one  meter.  Thus  the  work  of  the 
left  ventricle  at  each  contraction  is  130.5  grammetres  (or  15  foot 
pounds).  Add  45  grammetres  as  the  work  done  by  the  right 
ventricle  in  contracting.  If  the  heart  beats  72  times  per  minute 
it  will,  in  twenty-four  hours,  do  18,000  kilogramme-metres  of 
work. 


46  THE   CIRCULATION   OF   THE   BLOOD 

Sounds  of  the  Heart.— Listening  to  the  heart's  action  through 
the  thoracic  wall  we  hear  two  distinct  sounds.  The  first  is  a 
slightly  elongated  sound  and  comes  immediately  after  the  beat 
of  the  radial  pulse.  It  is  characterized  by  the  syllable  lub. 
The  cause  of  this  sound  is  supposedly  the  closure  of  the  auriculo- 
ventricular  valves  combined  with  the  sound  made  by  the  con- 
tracting muscle.  It  can  best  be  heard  over  the  apex  of  the 
heart. 

The  second  sound  is  shorter  and  sharper  than  the  first  and  is 
heard  just  before  the  impulse  of  the  radial  pulse.  It  is  char- 
acterized by  the  shorter  syllable  dap. 

The  cause  of  this  sound  is  supposedly  the  closure  of  the  aortic 
semi-lunar  valves  along  with  those  of  the  pulmonary  artery.  It 
is  best  heard  in  the  right  second  intercostal  space,  as  the  aortic 
current  transmits  it. 

Certain  diseases  affect  the  heart  valves  and  the  sounds  then 
depart  from  the  normal.  Thus  it  is  of  importance  to  know  the 
cause  and  sound  of  the  normal  vibrations  so  as  to  detect  the 
diseased  conditions. 

Heart  Innervation.-  The  nerves  that  inhibit  the  action  of 
the  heart  are  the  two  vagi ;  cutting  these  results  in  an  increase 
of  the  frequency  of  the  heart  beats. 

The  nerves  that  accelerate  the  action  of  the  heart  are  the 
nervi  accelerantes,  which  are  branches  of  the  sympathetic 
system.  Stimulation  of  these  causes  increase  in  force  and  fre- 
quency of  heart  beats. 

II.  CIRCULATION  IN  BLOOD-VESSELS. 

Taking  the  heart  as  a  central  station  for  supplying  force,  we 
find  the  blood  current  constantly  going  from  a  place  of  higher 
pressure  to  a  place  of  lower  pressure. 

The  highest  pressure  is  in  the  muscular  center,  the  heart. 
Blood-vessels  connect  with  both  auricles  and  ventricles.  Those 


CIRCULATION  IN   BLOOD-VESSELS  47 

connecting  with  the  ventricles  and  carrying  blood  away  from 
the  heart  are  called  arteries  and  the  pressure  in  these  is  high,  but 
lower  than  in  the  heart.  Those  vessels  connecting  with  the  auri- 
cles and  carrying  blood  back  to  the  heart  are  called  veins  and 
the  pressure  is  lowest  of  all  in  these. 

The  minute  vessels  that  connect  the  arteries  and  veins  and 
collect  waste  from  and  supply  nutritive  material  to  the  lymph 
stream  are  called  capillaries.  The 
pressure  in  these  is  lower  than  in  the 
arteries  but  higher  than  in  the  veins. 

The  blood  is  thus  kept  in  motion, 
constantly  going  from  place  of  higher 
to  lower  pressure.  FIG.  23. — Tracing  of  blood 

The  completed  circulation  is  thus: —     pressure  taken  with  Prick's 

(Beginning  with  the  right  auricle  of  manometer-  (Y™^ 
the  heart.)  The  two  venae  cavse  pour  venous  blood  into  the  right 
auricle  and  it  in  turn  empties  its  contents  into  the  right  ventricle. 
From  here  the  blood  is  driven  into  the  pulmonary  artery  (carrying 
venous  blood)  to  be  aerated  in  the  lungs.  From  the  lungs  it  comes 
by  pulmonary  veins  (carrying  arterial  blood)  to  the  left  auricle. 
This  is  the  lesser  or  pulmonary  circulation. 

From  the  left  auricle  the  blood  goes  into  the  left  ventricle  and 
from  here  it  is  forced  into  the  aorta  and  thus  into  the  systemic 
arteries,  then  through  the  capillaries  to  the  veins  and  back  by 
means  of  the  venae  cavae  into  the  right  auricle. 

The  complete  cycle  in  man  takes  about  twenty-two  seconds. 

STRUCTURE  OF  THE  BLOOD-VESSELS. 

Arteries. — The  arteries  have  three  coats:  (i)  the  external  coat 
called  the  tunica  adventitia,  which  is  composed  of  fibrous 
tissue  with  a  little  plain  muscular  tissue;  (2)  middle  coat  or 


THE    CIRCULATION    OF   THE    BLOOD 


tunica  media,  composed  of  yellow,  elastic  tissue;  and  (3)  the 
inner  coat  or  tunica  intima,  composed  of  epithelium  on  base- 
ment membrane. 

Veins. — The  veins  also  have 
three  coats,  the  external,  middle 
and  internal,  as  the  arteries;  but 
the  middle  coat  is  composed  chiefly 
of  inelastic,  fibrous  tissue.  Thus 
the  veins  lack  the  elasticity  and 
contractility  given  to  the  arteries  by 
the  middle  coat. 

The  Capillaries. — As  the  arteries 
get  smaller  we  find  them  still  com- 
posed of  the  three  above  named 
coats.  Finally,  though,  in  the 
minutest  vessels  we  find  only  the 
innermost  layer  remaining.  These 
one-coated  vessels  are  the  capillaries, 
and  they  have  only  one  layer  of 
epithelial  cells  on  a  basement  mem- 
brane. This  is  in  order  to  render 
possible  the  interchange  of  material 
between  the  blood  current  and  the 
lymph  stream,  so  the  tissues  may 

be  nourished  and  the  waste  prod- 
FIG.  24 -Scheme  of  the  uctg  removed> 

circulation. 

a,  right,  b,  left,  auricle;  A,  right, 
B,   left,   ventricle;    i,   pulmonary 


IMPORTANCE  OF  ARTERIAL 
ELASTICITY. 

If  an  amount  of  fluid  corre- 
sponding to  that  of  the  "  pulse 
volume"  be  suddenly  injected  into 
the  end  of  a  rubber  tube  already  distended  with  liquid,  the 
tube  will  be  further  distended  by  the  liquid  injected ;  but  if  a 


artery;  2,  aorta;  i,  area  of  pulrno 
nary,  K,  area  of  systemic,  circula- 
tion; o,  the  superior  vena  cava;  G, 
area  supplying  the  inferior  vena 
cava;  u;  d,  d,  intestine;  m,  mesen- 
teric  artery;  q,  portal  vein;  L,  liver; 
h,  hepatic  vein.  (Landois.) 


IMPORTANCE    OF   ARTERIAL    ELASTICITY  49 

like  amount  of  fluid  be  allowed  to  escape  at  the  other  end  the 
tube  will  resume  its  original  caliber.  Thus  the  pulse  volume 
enters  with  much  force  the  aorta  or  pulmonary  artery;  the  artery 
is  very  elastic  and  expands  under  this  influence,  but  immedi- 
ately recoils  with  a  great  pressure  on  the  contents.  The  pressure 
tends  to  force  the  blood  along  the  vessel  in  both  directions, 
but  its  return  into  the  ventricle  is  effectually  prevented  by  the 
close  of  the  semi-lunar  valves.  Consequently  it  can  go  only 
toward  the  periphery. 


FIG.  25. — Transverse  section  of  part  of  the  wall  of  the  posterior  tibial  artery. 
(Man.)     (From  Yeo  after  Shafer.} 

a,  endothelium  lining  the  vessel,  appearing  thicker  than  natural  from  the  contrac- 
tion of  the  outer  coats;  b,  the  elastic  layer  of  the  intima;  c,  middle  coat  composed  of 
muscle  fibers  and  elastic  tissue;  d,  outer  coat  consisting  chiefly  of  white  fibrous  tissue. 

Now  it  is  evident  that  the  flow  in  the  beginning  of  the  aorta 
is  intermittent;  but  it  is  found  that,  in  vessels  as  large  as  the 
carotids  the  flow  has  resumed  a  remittent  character.  The 
smaller  the  vessel  the  nearer  the  flow  becomes  continuous  until 
this  condition  is  established  in  the  capillaries. 

It  is  the  elastic  coat  of  the  arteries  that  allows  them  to 
expand  and  then  contract  on  the  contents  forcing  them 
onward.  Furthermore  it  is  this  elasticity  that  causes  the  inter- 
mittent and  remittent  flow  to  become  continuous.  So  the 
function  of  the  elastic  coat  is  two-fold;  first,  it  forces  the  blood 
current  continuously  toward  the  periphery,  and  second,  it  is 
chiefly  the  cause  of  the  change  from  an  intermittent  flow  to  a 
constant  flow,  which  is  of  so  much  importance  in  the  capillaries. 

4 


50  THE    CIRCULATION    OF   THE   BLOOD 

Rate  of  Flow. — The  velocity  of  the  blood  current  is  equal 
to  the  volume  flowing  through  a  determined  section  in  one 
second  divided  by  the  cross  section.  The  rate  is  determined  by 
the  pressure,  the  friction  in  the  vessels,  and  the  cross  section  of 
the  vessels. 

The  combined  cross  section  of  the  capillaries  is  greater  than 
the  combined  cross  section  of  the  arteries  or  the  veins,  so  the  rate 
of  flow  must  be  greater  in  the  arteries  and  veins  than  in  the  cap- 
illaries. The  friction  is  greater  in  the  smaller  vessels  than  in  the 
larger  which  retards  the  flow.  The  pressure  is  greater  in  the 
arteries  than  in  the  capillaries  and  veins.  From  these  facts  it  is 
evident  that  the  velocity  is  greater  in  the  arteries  than  in  the 
capillaries  and  veins,  but  increases  in  the  veins  as  compared  to 
the  capillaries. 

In  the  large  arteries  the  rate  is  200-400  m.m.  per  second,  in  the 
capillaries,  6-8  mm.  and  in  the  large  veins  it  is  but  little  less  than 
in  the  arteries. 

Valves  in  the  Veins. — At  frequent  intervals  in  the  course  of 
the  veins  are  found  small  folds  of  membrane  protruding  into  the 
lumen  of  the  vessels.  The  flow  of  the  blood  in  the  veins  is  more 
sluggish  than  in  the  arteries,  because,  as  we  have  seen,  the  pres- 
sure lessens  in  the  veins  while  gravity  and  friction  tend  to  cause  a 
stoppage.  These  protruding  folds  of  the  mucous  membrane  or 
valves  found  in  the  veins  aid  in  the  circulation  by  overcoming 
gravity  and  preventing  a  backward  flow  of  blood,  by  holding 
the  blood  until  a  fresh  impulse  can  impel  it  forward.  They  are 
found  in  pairs  and  are  most  abundant  in  the  veins  of  the 
extremities  where  gravity  impedes  the  onward  flow  of  the 
current. 

Capillary  Importance. — The  capillaries  are  the  smallest 
blood-vessels  and  the  most  important  as  to  function.  Being  of 
only  one  thickness  of  epithelium  and  in  direct  touch  with  the 
lymph  flow,  we  can  readily  see  that  the  food  products  brought  by 
the  arterial  blood  can  be  exchanged  here  for  waste  brought  bv 


INNERVATION    OF    VESSELS  51 

the  lymph.  The  flow  in  the  capillaries  is  constant,  as  we  have 
seen,  and  we  can  see  the  importance  of  this  when  we  take  into 
consideration  how  rapidly  the  tissues  use  oxygen  and  how 
necessary  is  a  constant  increasing  supply,  and  how  essential  it  is 
to  remove  the  carbon  dioxide  poisons. 

Innervation  of  Vessels. — The  blood-vessels  are  controlled 
by  the  sympathetic  nervous  system  by  means  of  the  vaso-motor 


FIG.  26. 

A,  vein  with  valves  open.     B,  with  valves  closed;  stream  of  blood  passing  off  by 
lateral  channel.     (Kirkes  after  Dalton.) 

nerves.  These  compose  both  the  vaso-constrictors,  or  the 
nerves  causing  the  vessels  to  contract,  and  the  vaso-dilators, 
those  causing  the  vessels  to  dilate.  The  entire  physiological  dis- 
tribution of  blood  is  regulated  by  the  vaso-motor  system  of  nerves. 
It  is  by  their  means  that  the  blood  is  increased  to  all  parts  of  the 
body  where  physiological  activity  is  going  on,  as  when  the  gastro- 
intestinal tract  is  active  during  digestion,  a  muscle  in  motion,  or 
glands  in  activity.  Paralysis  of  the  vaso-constrictors  causes 
blushing,  paralysis  of  the  dilators  causes  pallor  as  from  fright. 


52  THE    CIRCULATION   OF   THE    BLOOD 

Outside  influences  will  cause  the  constrictors  to  act,  as  cold; 
while  alcohol  will  cause  dilators  to  act  and  paralyzes  the 
constrictors. 

The  chief  vaso-motor  center  is  in  the  medulla  oblongata, 
while  subordinate  centers  exist  in  the  cord.  The  vaso-motor 
fibers  reaching  the  vessels  proceed  from  ganglia  in  the  sympa- 
thetic system,  but  these  ganglia  are  influenced  by  the  cells  in  the 
vaso-motor  center. 


FIG.  27. — Capillaries. 

The  outlines  of  the  nucleated  endothelial  cells  with  the  cement  blackened  by  the 
action  of  silver  nitrate.      (Landois.) 

Amount  of  Blood  Important.— When  there  are  small  losses 
of  blood  from  slight  injuries  the  entire  vascular  system  contracts 
and  the  current  supplying  this  diminished  area  is  sufficient; 
but  at  times  the  loss  of  blood  is  so  great  that  the  amount  remain- 
ing is  not  sufficient  to  carry  on  a  complete  circulation.  Unless 
remedied  this  results  in  death.  In  such  cases  of  great  loss  the 
deficit  may  be  supplied  by  a  normal  salt  solution,  thus  giving  an 
amount  of  fluid  sufficient  to  maintain  the  heart  action.  But  in 


PULSE 


53 


cases  where  as  much  as  two-thirds  of  the  blood  is  lost,  the  injec- 
tion of  fluid  does  no  good.  The  amount  to  cause  the  heart's 
action  to  continue  may  be  supplied,  but  the  amount  of  hemoglo- 


FIG.  28. — Interior  of  right  auricle  and  ventricle  exposed  by  the  removal  of  a 
part  of  their  walls.     (From  Yeo  after  Allen  Thompson.} 

inner  wall 
flaps  of 

been  cut; 

7,  on  aorta  near  the  ductus  arteriosus;  8,  9,  aorta  and  its  branches;  10,  u,  left  auricle 
and  ventricle. 


As- 


i,  superior  vena  cava;  2,  inferior  vena  cava;  2',  hepatic  veins;  3,  3',  3",  inr 
of  right  auricle;  4,4,  cavity  of  right  ventricle;  4',  papillary  muscle;  5,  5',  s"> 
tricuspid  valve;  6,  pulmonary  artery  in  the  wall  of  which  a  window  has  be 


bin  necessary  for  life  is  lost  and  this  cannot  be  supplied, 
phyxiation  is  the  result. 

Pulse.— If  a  finger  be  placed  on  any  artery  in  the  body  there 


54  THE    CIRCULATION    OF   THE    BLOOD 

will  be  transmitted  to  it  a  perceptible  impulse.     This  impulse  is 
what  is  called  the  pulse.     It  is  caused  by  the  force  of  the  heart's 


FIG.  29. — The  left  auricle  and  ventricle  opened  and  part  of  their    walls 
removed  to  show  their  cavities.     (From  Yeo  after  Allen  Thompson.} 

i,  right  pulmonary  vein  cut  short;  i',  cavity  of  left  auricle;  3,  3",  thick  wall  of 
left  ventricle;  4,  portion  of  the  same  with  papillary  muscle  attached;  5,  the  other 
papillary  muscles;  6,  6',  the  segments  of  the  mitral  valve,  7,  in  aorta  is  placed  over 
the  semi-lunar  valves. 

action  against  the  elastic  arterial  wall,  and  the  subsequent  con- 
traction of  this  wall  against  the  current  it  contains. 

The  impulse  is  an  index  to  the  condition  of  the  circulation. 


PULSE 


55 


Its  frequency  normally  in  an  adult  is  about  72  times  per  minute, 
in  children  it  is  higher,  and  it  is  more  frequent  in  woman  than  in 
man.  Its  frequency  is  affected  by  age,  sex,  exercise,  disease, 
drugs,  and  psychical  causes,  as  fear,  joy,  sorrow,  etc.  We  feel 


FIG.  30. — Portion  of  the  wall  of  ventricle. 

d,  d' ,  and  aorta,  a,  b,  c,  showing  attacHments  of  one  flap  of  mitral  and  the  aortic 
valves;  h  and  g,  paoillary  muscles;  e,  e'  and/,  attachment  of  the  tendinous  cords. 
(From  Yeo  after  Allen  Thompson.) 


the  pulse  to  learn  several  things: — (i)  Its  frequency,  which  tells 
how  many  times  the  heart  is  beating. 

(2)  Its  tension,  which  is  the  state  of  the  arterial  walls  and  is 


50  THE    CIRCULATION    OF   THE    BLOOD 

the  resistance  offered  in  peripheral  vessels.     We  judge  the  ten- 
sion by  the  force  necessary  to  obliterate  the  impulse. 

(3)  Regularity,  which  tells  whether  the  heart  is  regular  in 
either  its  force  or  rhythm. 

(4)  Its  strength,  which  tells  as  to  the  force  with  which  the 
heart  is  acting. 

(5)  Its  length,  whether  the  beat  is  long  or  slow  and  continuous. 


FIG.  31. — Dudgeon  sphygmograph. 


(6)  The  condition  of  the  vessel  wall,  whether  sclerotic  or  not. 
In  the  study  of  the  pulse  an  instrument  called  the  sphygmograph 
is  used,  which  receives  the  impulse  from  a  beating  artery  and 
transmits  it  by  means  of  a  finely  adjusted  lever  to  a  smoked  sur- 
face of  paper.  Thus  a  graphic  representation  of  the  impulse  is 
given,  the  height  to  which  the  writing  end  of  the  lever  goes  denot- 
ing the  force  of  the  impulse  of  the  heart  beat  at  the  time  of  the 
writing. 


THE    LYMPH  5  7 

THE  LYMPH. 

The  lymph  is  a  clear  colorless  fluid  contained  in  the  lymphatic 
vessels  and  tissue  spaces.  It  resembles  plasma  in  general 
appearance  and  does  not  differ  greatly  from  it  in  composition. 

The  Lymph  Vessels. — These  vessels  originate  in  at  least 
three  different  ways,  (i)  All  cells  may  be  said  to  be  bathed  in 
lymph,  being  surrounded  by  that  fluid  lying  in  the  irregularly 
shaped  spaces  between  them.  These  spaces  communicate  with 
each  other  and  finally  converge  to  the  lymph  capillaries.  The 
intervals  are  called  the  "  extravascular  lymph  spaces."  (2)  In 
certain  situations,  particularly  in  the  nervous  centers,  the  small 
blood-vessels  are  completely  surrounded  by  and  included  in 
larger  tubes,  the  (lperivascu!ar  lymph  canals."  These  likewise 
pass  on  to  the  lymph  capillaries  proper.  (3)  The  large  serous 
cavities,  like  those  lined  by  the  peritoneum,  pleura,  tunica  vagin- 
alis,  etc.,  have  large  numbers  of  lymphatic  radicles  opening 
abruptly  into  them,  or  rather  originating  from  them,  and  these 
may  be  considered  as  great  extravascular  lymph  spaces. 

The  course  of  the  lymph  is  from  the  tissues  to  the  subclavian 
veins,  where  it  enters  the  vascular  circulation.  The  lymphatic 
vessels  from  the  right  arm  and  the  right  side  of  the  face,  head  and 
chest  converge  to  form  the  ductus  lymphaticus  dexter,  which 
enters  the  right  subclavian  vein  at  its  junction  with  the  internal 
jugular.  The  lymphatics  from  all  other  parts  of  the  body  con- 
verge to  form  the  thoracic  duct,  which  enters  the  left  subclavian 
vein  at  its  junction  with  the  internal  jugular.  The  thoracic 
duct  begins  by  a  dilated  pouch  lying  upon  the  second  lumbar 
vertebra.  This  pouch  receives  the  lymphatic  branches  which 
have  converged  from  the  lacteals,  and  is  called  the  recep- 
taculum  chyli.  The  lacteals  pass  through  the  mesenteric  lym- 
phatic glands  on  their  way  to  the  receptaculum  chyli. 

The  distribution  of  the  lymphatics  needs  no  comment  when 
it  is  known  that  they  receive  the  plasma  which  has  been  passed 


THE   CIRCULATION    OF   THE   BLOOD 


FIG.  32. — Diagram  showing  the  course  of  the  main  trunks  of  the  absorbent 

system. 

The  lymphatics  of  lower  extremities,  D,  meet  the  lacteals  of  intestines,  LAC,  at 
the  receptaculum  chyli,  R.C.  .where  the  thoracic  duct  begins.  The  superficial  vessels 
are  shown  in  the  diagram  on  the  right  arm  and  leg,  S,  and  the  deeper  ones  on  the  left 
arm,  D.  The  glands  are  here  and  there  shown  in  groups.  The  small  right  duct 
opens  into  the  veins  on  the  right  side.  The  thoracic  duct  opens  into  the  union  of  the 
great  veins  of  the  left  side  of  the  neck,T.  (Yeo.) 


THE    LYMPHATIC    GLANDS  59 

out  of  the  vascular  capillaries  and  thus  collect  fluid  from  well- 
nigh  every  tissue  in  the  body. 

The  structure  of  the  lymph-vessels  is  quite  similar  to  that  of 
the  veins,  though  they  are  more  delicate.  The  lymph  capillaries 
probably  contain  only  a  single  coat  like  the  venous  capillaries. 
In  the  large  vessels  this  thin  endothelial  coat  is  supplemented  by 
connective  tissue  fibers  together  with  some  elastic  and  non- 
striated  muscle  fibers.  They  are  very  abundantly  supplied 
with  valves  which  operate  in  the  same  way  as  the  venous  valves: 
The  vessel  wall  is  quite  elastic  and  has  some  contractile  power. 

Lymphatic  Glands. — All  the  lymphatics  pass  through  one  or 
more  lymphatic  glands  on  their  way  to  the  larger  trunks.  These 
bodies  are  not  true  glands.  Their  structure  is  adenoid.  There 
are  some  six  or  seven  hundred  in  the  body,  varying  in  size  from 
a  pinhead  to  a  large  bean.  The  superficial  glands  are  especially 
abundant  about  the  groin,  axilla,  neck  and  the  other  flexures. 
The  deep  ones  are  most  numerous  about  the  great  vessels.  The 
mesenteric  glands  are  found  between  the  folds  of  the  mesentery. 

The  lymphatic  glands  are  of  irregular  shape  and  contain 
within  their  substance  large  numbers  of  lymph  spaces  or  canals 
through  which  the  incoming  lymph  must  pass.  The  vasa 
efferentia  are  usually  fewer  in  number  and  larger  in  size  than  the 
vasa  qfferentia.  The  current  must  be  considerably  delayed  in 
the  glands.  They  are  probably  concerned  in  the  elaboration  of 
leucocytes  of  the  lymphatic  circulation,  while  their  retention  of 
toxic  materials — even  to  their  own  hurt — is  a  common  patho- 
logical occurrence. 

Properties  and  Composition  of  Lymph. — Lymph  is  a  com- 
paratively clear  liquid  containing  leucocytes.  After  meals  the 
color  becomes  whitish  from  the  admixture  of  chyle,  and  numerous 
fat  droplets  are  present.  Neither  red  corpuscles  nor  platelets 
are  thought  to  be  found  in  lymph  except  accidentally.  The 
specific  gravity  is  lower  than  that  of  the  blood.  Lymph  coagu- 
lates when  drawn,  since  the  fibrin  factors  are  present;  but  the 


60  THE    CIRCULATION    OF   THE    BLOOD 

process  is  less  prompt  and  the  clot  is  less  firm  than  in  the  case 
of  blood. 

In  order  to  form  an  idea  as  to  the  constituents  of  lymph  it  is 
only  necessary  to  say  that  its  ultimate  origin  is  the  blood  plasma, 
except  in  so  far  as  its  composition  is  changed  during  digestion. 
The  plasma  makes  its  way  through  the  capillary  walls  out  to  the 
tissues  bringing  nourishment  to  them  and  removing  waste 
products  from  them.  In  thus  coming  in  contact  with  the  tissues 
the  plasma  finds  itself  in  the  extravascular  lymph  spaces  and  its 
name  is  simply  changed  to  lymph.  It  thus  appears  that  lymph 
may  enter  the  extravascular  spaces  by  the  direct  passage  of 
plasma  out  of  the  vessels  or  by  being  excreted,  as  it  were,  from 
the  tissue  cells. 

In  any  case  the  constituents  of  lymph  are  not  very  different 
from  those  of  plasma,  except,  of  course,  when  intestinal  digestion 
is  in  progress  and  chyle  is  introduced  into  the  lymphatic  circula- 
tion. It  contains  the  three  plasma  proteids,  urea,  fat,  lecithin, 
cholesterin,  sugar  and  inorganic  salts.  The  proteids  are  less 
abundant  than  in  plasma,  as  might  be  supposed  when  it  is 
remembered  that  they  possess  little  osmotic  power.  The 
inorganic  salts  are  in  about  the  same  proportion  in  both  fluids. 
It  is  significant  that  the  amount  of  urea  and  related  excrementi- 
tious  products  is  more  abundant  in  lymph  than  in  plasma;  their 
source  is  the  destructive  metabolism  going  on  in  the  cells  to 
which  the  plasma  has  been  supplied,  this  plasma  finding  its  way 
back  as  lymph.  It  is  by  no  means  certain,  however,  that  all  the 
plasma  escaping  from  the  capillaries  is  carried  away  by  the 
lymphatic  system.  Some  may  reenter  the  blood-vessels. 

There  is  no  unanimity  of  opinion  as  to  the  exact  method  of 
passage  of  plasma  through  the  capillary  walls  into  the  lymph 
spaces.  Some  maintain  that  the  phenomena  can  be  explained 
by  the  ordinary  physical  laws  of  diffusion,  filtration  and  osmosis 
when  existing  conditions  of  pressure,  etc.,  are  taken  into  consid- 
eration. Others  hold  that  these  laws  are  insufficient  in  them- 


THE    FLOW   OF   LYMPH  6 1 

selves  to  account  for  various  occurrences  in  this  connection,  and 
ascribe  to  the  capillary  endothelium  some  active  secretory  power 
governing,  or  at  least  influencing,  the  outward  passage  of  the 
plasma. 

The  Flow  of  Lymph. — There  is  no  organ  corresponding  to 
the  heart  to  keep  the  lymph  current  in  motion.  The  main  causes 
for  its  direction  from  the  extravascular  spaces  toward  the  veins  in 
the  neck  is  the  degree  of  pressure  to  which  it  is  subjected  in  those 
spaces  as  compared  with  the  inferior,  or  even" negative,"  pres- 
sure obtaining  near  the  terminations  of  the  great  ducts.  It  is 
known  that  at  all  times  the  venous  pressure  in  the  subclavian 
veins  is  low  and  that  it  may  even  fall  below  the  atmospheric  pres- 
sure, so  that  "suction"  is  exerted  upon  the  lymphatic  ducts  where 
they  enter  those  vessels.  The  lymph  pressure  in  the  extravas- 
cular spaces  is  estimated  to  be  one-half  the  capillary  blood-pres- 
sure. Friction  and  gravity  (where  the  course  of  the  vessels  is 
upward)  oppose  the  passage  of  the  fluid.  Consequently  it  ac- 
cumulates in  the  spaces  and  in  the  smaller  lymphatics  untU  the 
pressure  there  becomes  greater  than  the  resistance  of  these  forces, 
when  it  passes  onward.  Since  lymph  is  being  continually  pro- 
duced this  superior  pressure  in  the  extravascular  spaces  and  small 
lymphatics  is  a  fairly  constant  factor  and  keeps  up  a  correspond- 
ingly constant  current. 

There  are  two  factors  which  are  accessory  to  this  peripheral 
pressure:  (i)  Thoracic  aspiration  by  bringing  about  negative 
pressure  in  the  veins  in  and  near  the  chest  brings  about  a  like 
condition  in  the  tributary  lymphatic  ducts;  furthermore,  the 
effect  of  aspiration  makes  itself  felt  directly  upon  the  thoracic 
duct  since  its  greatest  extent  is  in  the  thorax.  (2)  The  valves  of 
the  lymphatics  act  in  a  similar  manner  to  those  of  the  veins  and 
constitute  a  very  necessary  factor  in  the  lymphatic  circulation. 
Although  the  lymph  flow  resembles  that  of  the  venous  blood,  it  is 
less  regular  and  more  sluggish,  but  probably  not  so  slow  as  might 
be  supposed.  Properly  colored  solutions  injected  into  the  blood 


62  THE   CIRCULATION    OF   THE   BLOOD 

have  been  demonstrated  in  the  lymph  of  the  thoracic  duct  "in 
from  four  to  seven  minutes." 

Lymph  and  Chyle. — It  is  scarcely  necessary  to  refer  to  the 
differences  between  these  two  fluids.  Chyle  is  the  intestinal 
lymph  during  digestion.  In  the  intervals  of  digestion  the  con- 
tents of  the  lacteals  do  not  differ  materially  from  lymph  in  other 
localities.  Chyle  has  a  whitish  milky  appearance  due  to  the 
presence  of  emulsified  and  saponified  fats.  Its  specific  gravity 
naturally  depends  largely  upon  the  amount  of  fat  ingested,  but 
is  always  higher  than  that  of  ordinary  lymph  and  lower  than 
that  of  blood.  Not  only  is  there  more  fat  in  the  chyle  than  in 
lymph,  but  the  other  solids  are  also  increased.  The  proteid  con- 
stituents are  considerably  more  abundant.  For  the  most  part 
the  higher  specific  gravity  is  explained  by  the  absorption  of  solids 
in  solution  from  the  alimentary  canal. 

Chyle  is  forced  out  of  the  lacteals  by  contraction  of  the  non- 
striated  muscle  fibers  which  run  along  by  the  vessel.  When  re- 
laxation of  the  fibers  occurs,  return  of  chyle  into  the  lacteal  is 
prevented  by  a  valve  at  the  base  of  the  villus. 


CHAPTER  VII. 

THE  PHYSIOLOGY  OF  DIGESTION  AND 
ABSORPTION. 

FOODS. 

IT  is  evident  that  all  the  tissues  of  the  body  are  continually 
undergoing  "physiological  wear" — that  the  materials  of  which 
they  are  intrinsically  composed  are  being  changed  into  effete 
matter  and  discharged  from  the  system.  This  is  a  process  going 
on  in  the  substance  of  every  cell  in  the  body,  and  obviously,  for 
these  cells  to  continue  to  live  and  functionate,  there  must  be  a 
continual  appropriation  of  new  matter  to  take  the  place  of  the 
materials  which  have  served  their  physiological  purpose,  and  are 
of  no  further  value  to  the  body.  This  supply  of  material  is  made 
directly  to  the  tissues  by  the  blood,  but  lest  this  fluid  be  impover- 
ished, it  must  in  turn  be  furnished  with  an  approximately  constant 
quantity  of  nutritive  matter.  The  ultimate  source  of  that  matter 
is  in  the  food  which  we  eat.  However,  it  must  pass  through  the 
processes  of  digestion  and  absorption  before  it  can  be  utilized 
by  the  tissues.  This  conception  of  a  food  must  be  understood 
to  embrace  all  substances  contributing,  either  directly  or  indi- 
rectly, to  body  nutrition,  including,  therefore,  the  oxygen  of  the 
air  as  well  as  all  articles  usually  classed  as  drinks. 

An  animal  whose  weight  remains  about  the  same  must  eat  and 
digest  a  certain  quantity  of  food  to  keep  up  the  body  temperature, 
to  supply  mechanical  energy,  and  to  repair  the  wastes  which  are 
continually  going  on  in  the  body.  An  animal  which  is  growing 
and  increasing  in  weight  must  eat  enough  not  only  to  supply  the 
demands  just  mentioned,  but  also  to  form  the  new  tissue. 

The  articles  we  eat,  besides  being  largely  insoluble,  differ  very 
materially  in  their  composition  from  any  substances  found  as 

63 


64  THE   PHYSIOLOGY    OF   DIGESTION  AND   ABSORPTION 

parts  of  the  body  tissues.  Even  those  undigested  substances 
most  closely  resembling  living  tissue  will  not  be  utilized  by  the 
cells  when  presented  to  them  by  being  injected  into  the  blood.  All 
the  articles  which  we  use  for  food  must  undergo  a  special  process, 
called  digestion,  before  they  can  be  absorbed  by  the  tissues. 

Seat  of  Hunger. — Food  is  taken  into  the  body  in  obedience 
to  an  expressed  want  on  the  part  of  the  system.  The  desire  for 
food — the  sensation  of  hunger — is  referred,  in  a  rather  indefinite 
way,  to  the  stomach.  That  sensation  is  ordinarily  satisfied  by 
the  introduction  of  food  into  the  stomach.  However,  this  does 
not  necessarily  mean  that  its  seat  is  in  that  organ,  since  removal 
of  the  stomach  by  no  means  prevents  hunger.  But,  if  nutritious 
material  be  introduced  in  sufficient  quantity  into  the  circulation, 
as  by  rectal  enemata,  hunger  is  relieved.  The  true  seat  of  this 
sensation  is  undoubtedly  in  the  cells  themselves,  it  being  simply 
a  call  from  them  for  more  material  to  take  the  place  of  their 
worn-out  constituents. 

Cold  weather  demands  an  increase  in  the  amount  of  food,  as 
also  do  physical  and  psychical  activity,  certain  drugs,  etc. 

Seat  of  Thirst. — The  demand  of  the  cells  for  water  is  referred 
to  the  fauces  and  throat,  but  this  is  no  more  the  seat  of  thirst 
than  is  the  stomach  of  hunger.  The  taking  of  water  into  the 
mouth  alone  will  not  quench  thirst,  except  in  so  far  as  absorp- 
tion may  take  place  from  its  mucous  membrane.  But,  if  water 
in  sufficient  amount  be  placed  into  the  circulation  in  any  way 
satisfaction  ensues.  Next  to  the  demand  for  oxygen,  that  for 
water  is  the  most  imperative  which  comes  from  the  tissues ;  that 
is,  they  can  live  much  longer  without  solid  food  than  without 
water.  The  amount  necessary  is  manifestly  subject  to  many  con- 
ditions, such  as  external  moisture  and  temperature,  exercise,  etc. 

Classification  of  Foods. — A  very  large  number  of  substances 
are  taken  into  the  alimentary  canal  as  food;  but  examination 
reveals  that  all  such  materials  contain  one  or  more  of  a  very 
few  classes  of  food  stuffs.  These  may  be  divided  as  follows: 


FOODS  65 

I.  Water. 

II.  Inorganic  or  mineral  salts. 

III.  Carbohydrates. 

IV.  Fats. 

V.  Proteids. 

I.  Water  is  scarcely  looked  upon  as  food  in  the  common 
acceptation  of  the  term,  but  it  is  quite  as  necessary  to  cell  life 
as  any  of  the  other  classes.     It  is  found  in  all  foods  and  in  all 
tissues  and  fluids  of  the  body.     It  forms  about  70  per  cent,  of 
the  entire  body  weight  and  acts  as  a  solvent  upon  various  ingre- 
dients of  the  food,  liquefying  them  and  rendering  them  capable 
of  absorption. 

II.  The  mineral  salts  which  are  chiefly  necessary  for  nutrition 
are: 


Of  sodium  and  potassium. 


Chlorides 
Phosphates 
Sulphates 
Carbonates 

Phosphates   j  . 

>   Of  calcium  and  magnesium. 
Carbonates  J 

Of  these  salts,  sodium  chloride,  or  common  table-salt,  is  the  most 
important  and  abundant  in  the  foods  we  eat.  It  is  present  in 
nearly  all  the  tissues  and  fluids  of  the  body,  especially  the  blood. 
Of  the  other  salts,  those  of  calcium  exist  in  the  largest  quantity 
in  the  body.  They  are  especially  important  on  account  of  the 
part  they  play  in  the  formation  of  the  bones,  teeth  and  cartilages. 
The  remaining  salts  exist  in  larger  or  smaller  quantities  in  the 
tissues  and  fluids  of  the  body. 

III.  The  carbohydrates  include  principally  the  starches  and 
sugars.  They  are  of  definite  chemical  composition  containing 
carbon,  hydrogen  and  oxygen,  but  no  nitrogen.  The  hydrogen 
and  oxygen  which  they  contain  are  always  in  the  proportion  to 
form  water,  i.  e.,  two  atoms  of  hydrogen  to  one  of  oxygen.  The 
5 


66  THE   PHYSIOLOGY   OF   DIGESTION  AND  ABSORPTION 

starches  are  found  chiefly  in  wheat,  corn,  oats  and  other  grains; 
also  in  potatoes,  peas,  beans  and  in  the  roots  and  stems  of 
many  plants,  and  in  some  fruits.  Starch  is  found  in  a  pure  state, 
as  a  white  powder,  in  arrowroot  and  corn-starch.  The  sugars 
are  of  several  kinds,  the  principal  being:  cane  sugar,  beet  sugar, 
maple  sugar,  grape  sugar  which  is  found  in  grapes,  peaches  and 
other  fruits,  and  malt  sugar  which  is  obtained  from  malt.  These 
are  all  obtained  from  vegetable  tissue,  however  a  few  are  found 
in  or  formed  by  the  animal  organism,  as  glycogen,  dextrose  and 
lactose.  They  are  the  cheapest  foods  from  financial  and  digest- 
ive standpoints  and  constitute  the  main  bulk  of  articles  eaten. 
They  contain  more  oxygen  than  do  the  fats,  and  are  more  easily 
oxidized  and  converted  into  heat  and  muscular  energy.  In 
fact,  their  great  physiological  value  lies  in  the  ease  with  which 
they  are  burned  up  in  the  body.  They  furnish  the  main  part 
of  the  fuel  necessary  to  the  running  of  the  animal  mechanism. 
They  may  also  be  converted  into  fatty  tissue  by  the  body. 

IV.  The  fats  are  ingested  with  both  animal  and  vegetable 
diets.     They  are  compounds  of  carbon,  hydrogen  and  oxygen. 
The  principal  fats  are  stearin,  palmatin,  margarin  and  olein. 
These  exist  in  varying  proportions  in  the  fat  of  animals,  in  the 
various  vegetable  oils  and  in  milk,  butter,  lard  and  in  other  foods 
and  vegetable  substances.     The  fats  contain  no  nitrogen,  and,  like 
carbohydrates,  their  great  physiological  value  lies  in  the  fact  that 
they  are  destroyed  in  the  organism  to  produce  energy,  whether 
in  the  form  of  heat  or  muscular  exercise.     They  are  handled  and 
converted  less  readily  by  the  system  than  the  carbohydrates,  and 
consequently  tax  the  digestive  powers  more.     But  it  is  found 
that,  weight  for  weight,  they  are  more  efficient  in  the  production 
of  energy  than  are  the  carbohydrates.     They  also  furnish  fuel 
for  the  running  of  the  body  mechanism. 

V.  The  proteids  form  a  large  part  of  all  living  organisms  and 
are  absolutely  necessary  to  animal  life.     They  are  very  stable 
compounds  and  are  found  in  both  animal  and  vegetable  foods. 


FOODS  67 

They  contain  carbon,  hydrogen,  oxygen  and  nitrogen,  together 
with,  usually,  a  small  quantity  of  sulphur  and  phosphorus. 
They  occur  in  the  form  of  casein  in  milk  and  cheese,  myosin  and 
syntonin  in  muscle,  vitellin  in  the  yolk  of  eggs,  glutein  in  flour, 
legumin  in  peas,  beans  and  lentils,  and  in  some  other  forms. 
Proteids  may  be  used  by  the  body  to  produce  heat  and  energy, 
but  being  more  stable  in  composition  than  carbohydrates  and 
fats,  they  are  more  often  used  to  build  up  tissue.  In  fact  the 
proteids  are  absolutely  essential  to  life  while  this  is  not  true  of 
carbohydrates  and  fats,  since  the  proteids  must  be  used  to  build 
up  new  cells  to  take  the  place  of  those  being  constantly  worn  out 
and  eliminated. 

The  animal  foods  which  are  richest  in  proteids  are  lean  meat, 
milk,  eggs,  cheese  and  all  kinds  of  fish,  while  the  vegetable  are 
wheat,  beans,  peas  and  oatmeal.  It  has  been  found  that  the 
animal  proteid  foods  are  split  up  and  digested  much  more 
easily  than  are  the  vegetable.  Hence  the  great  majority  of  the 
people  rely  upon  the  animal  foods  for  their  supply  of  proteid 
material  which  is  necessary  to  life. 

The  composition  of  a  few  of  the  more  important  articles  used 
as  food  is  shown  by  the  following  tables.* 

Milk:  Woman,       Cow, 

Per  cent.   Per  cent. 

Protein  (chiefly  caseinogen) 1.7  3.5 

Butter  (fat) 3.4  3.7 

Lactose 6.2  4.9 

Salts o .  2  0.7 

Eggs: 

Total  amount  of  solid 13  .3  per  cent. 

Protein 12.2  per  cent. 

Sugar 0.5  per  cent. 

Fats ) 

Lecithin |  Traces. 

Cholesterin J 

Inorganic  salts 0.6  per  cent. 

*These  tables  are  taken  from  Halliburton's  Handbook  of  Physiology. 


68            THE   PHYSIOLOGY    OF  DIGESTION  AND   ABSORPTION 

Meats:  Ox.      Calf.     Pig.    Fowl.    Pike. 

Water 76.7     75.6     72.6     70.8     79.3 

Solids 23.3     24.4     27.4     29.2     20.7 

Proteins 20.0     19.4     19-9     22.7     18.3 

Fats 1.5       2.9      6.2       4.1       0.7 

Carbohydrates 0.6       0.8       0.6       1.3       0.9 

Salts 1.2       1.3       i.i       i.i       0.8 

Vegetable  Foods:  Wheat.  Barley.  Oats.    Rice.      Peas.    Potatoes. 


Water  

12 

6 

T? 

8 

12 

^ 

17 

T 

14 

8 

76  .0 

Protein  
Fat 

....        12 
I 

•4 

A 

ii 

2 

.1 

2 

IO 

e 

•  4 
2 

n 

o 

9 

n 

23 

I 

•7 

6 

2.0 
O    2 

Starch  

67 

o 

6/| 

Q 

C7 

8 

76 

B 

40 

?. 

20  .6 

Cellulose           .    .    . 

2 

r 

f. 

7, 

II 

0 

o 

6 

7 

e 

O    7 

Mineral  salts  .  . 

I 

8 

2 

.7 

3 

0 

T 

0 

3 

.T 

I    O 

DIGESTION. 

Object. — Digestion  is  largely  a  chemical  process.  Certain 
physical  phenomena  are  auxiliary.  The  foods  not  yielding  en- 
ergy are  not  affected  in  a  chemical  way  by  digestion.  They  are 
simply  dissolved,  if  not  already  in  solution,  and  are  discharged 
from  the  body  in  the  same  condition  in  which  they  entered.  But 
the  other  classes  of  food  must  either  be  separated  from  innutri- 
tious  substances  with  which  they  enter,  or  undergo  certain 
changes  themselves,  or  both,  before  they  can  be  absorbed  and 
assimilated.  This  necessitates  a  complicated  digestive  apparatus 
and  the  subjecting  of  different  classes  of  food  to  different  diges- 
tive fluids  and  other  gastro-intestinal  influences.  The  object  of 
digestion  is  therefore  twofold,  first,  to  convert  the  foods  into 
soluble  materials  and,  second,  to  bring  about  such  changes  in 
their  composition  as  will  insure  their  absorption  and  appropria- 
tion by  the  tissues. 

Enzymes. — The  chemical  changes  taking  place  in  digestion 
are  of  a  peculiar  nature,  in  that  they  are  effected  largely  by  the 
presence  of  substances  known  as  enzymes,  corresponding  in  an 


DIGESTION  69 

obscure  way  with  ordinary  chemical  reagents.  These  have  been 
called  unorganized  or  unformed  ferments,  to  distinguish 
them  from  such  organized  ferments  as  bacteria,  yeast,  fungi, 
etc.  They  are  not  themselves  possessed  of  any  vital  activity, 
though  formed  in  living  organisms,  like  plants  or  animals.  They 
are  of  indefinite  chemical  composition,  contain  nitrogen  and  are 
supposed  to  be  of  proteid  structure.  The  characteristic  point  in 
their  action  has  been  supposed  to  be  that  they  produce  a  chem- 
ical change  without  themselves  being  affected  by  that  change.  This 
is  doubtless  practically  true,  but  it  is  found  in  experimental  work 
that  "a  given  solution  of  enzyme  cannot  be  used  over  and  over 
again  indefinitely."  It  finally  loses  its  identity. 

According  to  the  foods  on  which  they  act  and  the  effects  they 
produce,  enzymes  are  classified  as:  (i)  Proteolytic  enzymes, 
which  convert  proteids  into  soluble  peptones;  examples  are 
pepsin  and  trypsin.  (2)  Amylolytic  enzymes,  which  convert 
starches  into  sugar;  examples  are  ptyalin  and  amytopsin.  (3) 
Fat-splitting  enzymes,  which  convert  neutral  fats  into 
glycerine  and  fatty  acids;  an  example  is  steapsin.  (4) 
Sugar-splitting  enzymes,  which  convert  the  non-absorbable 
(saccharose)  into  absorbable  (dextrose)  sugar;  an  example  is 
invertase.  (5)  Coagulating  enzymes,  which  precipitate  solu- 
ble proteids;  an  example  is  rennin. 

Characteristics  of  Enzymes. — Some  of  the  characteristics  of 
enzymes  are  as  follows:  (i)  They  are  soluble  in  water  and  in 
glycerine.  (2)  In  solution  they  are  destroyed  before  the  boiling 
point  is  reached  (140°  to  180°  Fahrenheit).  Very  low  temper- 
atures do  not  destroy  them,  but  suspend  their  action.  (3) 
They  never  completely  convert  the  substance  upon  which  they 
act.  It  is  supposed  that  the  substance  produced,  as  peptones  for 
example,  have  an  inhibitory  action  upon  the  enzyme.  If  these 
substances  be  removed  as  they  are  formed,  the  action  of  the 
enzyme  continues.  (4)  The  particular  result  is  independent  of 


70  THE   PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

the  amount  of  the  enzyme  (unless  it  be  very  small)  no  matter 
how  large  a  quantity  of  the  substance  to  be  acted  upon  is 
present. 

Manner  of  Action. — These  enzymes  are  supposed  to  bring 
about  their  respective  changes  through  hydrolysis — that  is,  by 
causing  water  to  be  taken  up  by  the  molecules  of  the  affected 
substance  and  by  the  subsequent  splitting  of  the  newly  formed 
molecule  into  two  or  more  simpler  ones.  How  they  cause  this 
appropriation  of  water  is  as  yet  undetermined.  It  was  formerly 
supposed  to  be  brought  about  by  contact  merely,  and  the  enzymes 
were  called  catalytics;  but  this  term  offers  no  explanation  of  the 
real  change  which  occurs. 

Digestive  Processes. — The  digestive  processes  may  be  con- 
sidered under  the  heads  of  (i)  prehension,  (2)  mastication,  (3) 
salivary  digestion,  (4)  deglutition,  (5)  gastric  digestion  and  (6) 
intestinal  digestion.  Prehension,  mastication  and  deglutition 
cannot  properly  be  looked  upon  as  digestive  processes,  inasmuch 
as  they  involve  no  chemical  change.  They  are,  however,  neces- 
sary occurrences,  and  cannot  be  disregarded.  Of  course,  ab- 
sorption and  " internal  digestion"  are  supposed  to  follow  gastro- 
intestinal or  "  external  digestion,"  and  assimilation  or  cell 
appropriation  to  follow  absorption. 

Prehension. 

Prehension  is  simply  the  taking  of  food  into  the  mouth.  Its 
mechanism  in  the  human  adult  is  so  familiar  that  it  needs  no 
description.  In  the  sucking  child  it  is  more  complex.  The 
buccal  cavity  is  closed  posteriorly  by  the  application  of  the  velum 
palati  to  the  base  of  the  tongue.  The  tip  of  the  tongue  is  applied 
to  the  hard  palate,  and  successive  portions  of  it  (going  backward) 
being  applied  in  the  same  way  leave  a  partial  vacuum  in  front, 
and  liquids  are  drawn  into  the  mouth.  The  mechanism  of 
drinking  is  the  same. 


SALIVARY   GLANDS  .  'JI 

Digestion  in  the  Mouth. 

Mastication. — The  object  of  mastication  is  to  grind  up  the 
food  so  that  it  may  be  swallowed  more  easily  and  the  various 
digestive  fluids,  particularly  the  saliva  and  gastric  juice,  may 
have  more  ready  access  to  its  parts.  The  proper  mastication 
of  the  food  is  an  important  factor  in  its  complete  digestion  later 
on. 

Mechanically,  mastication  is  effected  by  the  action  of  the 
lower  jaw,  aided  by  the  tongue,  lips  and  cheeks.  This  remark 
presumes  of  course  that  the  teeth  are  intact.  Lateral  and 
antero-posterior  movements  of  the  lower  jaw  combine  with  its 
simple  elevation  to  compress  and  grind  the  food  between  the 
teeth.  The  muscles  which  depress  the  lower  jaw  are  the  digas- 
tric, mylohyoid,  geniohyoid  and  platysma.  Those  which  elevate 
it  are  the  temporal,  masseter,  internal  and  external  pterygoids. 
The  attachments  of  the  external  pterygoids  are  such  that  by 
their  simultaneous  action  the  mandible  can  be  thrown  forward 
and,  by  their  alternate  contraction,  from  side  to  side.  The 
tongue  is  active  during  mastication  in  carrying  the  mass  of 
food  to  this  or  that  part  of  the  buccal  cavity  so  that  it  may  be 
ground  up  completely.  It  also  gives  accurate  information  as  to 
the  size  (of  the  mass)  and  stage  of  mastication.  The  cheeks,  as 
is  shown  in  facial  palsy,  are  quite  important  in  keeping  the  food 
from  between  them  and  the  teeth.  The  lips  prevent  the  escape 
of  liquids  from  the  mouth,  in  addition  to  assisting  in  prehension. 

The  Salivary  Glands  and  their  Secretion. 

The  first  of  the  digestive  juices  with  which  the  food  comes  in 
contact  is  the  saliva  which  is  the  mixed  secretion  of  the  large 
salivary  glands  and  the  various  smaller  mucous  and  serous 
glands  which  open  into  the  mouth  cavity.  The  chief  salivary 
glands  are  three  in  number  on  each  side  of  the  mouth — the 
parotid,  submaxillary  and  sublingual.  Besides  these,  there 


72  THE    PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

are,  throughout  the  buccal  mucous  membrane,  a  number  of 
smaller  glands  of  similar  structure  contributing  to  the  formation 
of  saliva.  The  parotid  gland  is  situated  just  beneath  and  in 
front  of  the  lobe  of  the  ear;  the  submaxillary  beneath  the 
mandible  about  the  center  of  the  base  of  the  submaxillary  tri- 
angle, and  the  sublingual  beneath  the  mucous  membrane  of  the 
mouth,  just  lateral  to  the  lingual  frenum. 


FIG.  33. — Cells  of  the  alveoli  of  a  serous  or  watery  salivary  gland.     (Brubaker 

after  Yeo.) 

A,  after  rest;  B,  after  a  short  period  of  activity;  C,  after  a  prolonged  period  of 
activity. 

The  duct  from  the  parotid,  Stenson's  duct,  runs  beneath  the 
mucous  membrane  of  the  cheek  to  a  point  opposite  the  second 
upper  molar  tooth,  where  is  its  opening  into  the  mouth.  The 
duct  from  the  submaxillary,  Wharton's  duct,  discharges  the 
secretion  from  that  gland  into  the  mouth  by  the  side  of  the  fre- 
num of  the  tongue.  The  secretion  from  the  sublingual  reaches 
the  mouth  by  a  number  of  small  ducts  (Rivinus)  which  open 
also  by  the  side  of  the  frenum,  and  sometimes  as  well  by  a  larger 
duct,  Bartholin's,  which  runs  parallel  with  Wharton's  and 
empties  near  it. 

Histology. — In  structure  the  salivary  glands  have  been  shown 
to  be  of  the  compound  tubular  variety,  the  secreting  part  being 
tubular.  The  parotid  is  a  serous  gland,  the  other  two  are  usually 
said  to  be  mucous,  though  they  contain  both  serous  and  mucous 
cells.  The  ducts  subdivide  into  smaller  ducts  and  tubes,  until  a 
distinct  tubule  is  distributed  to  every  acinus  and  becomes  the 


SALIVARY   GLANDS  73 

lumen  of  that  acinus.     The  whole  arrangement  resembles  the 
branchings  of  a  tree. 

The  flow  from  these  glands  is  greatly  increased  by  mastica- 
tion. From  the  parotid  the  flow  is  much  more  abundant  on  that 
side  upon  which  mastication  takes  place.  During  activity  it 
can  be  shown  that  the  granules  of  the  serous  cells  accumulate 
toward  the  lumen  of  the  acinus  while  the  outer  segment  of  the 
cells  becomes  comparatively  clear.  It  is  supposed  that  this  is 
an  essential  step  in  the  production  of  the  organic  constituents  of 


FIG.  34. — Section  of  a  mucous  gland.     (Brubaker  after  Lavdowsky.) 
A,  in  a  state  of  rest;  B,  after  it  has  been  for  some  time  actively  secreting. 

the  secretion — that  the  granules  contain  either  the  ptyalin  or  the 
substance  necessary  to  its  formation.  It  is  also  supposed  that  at 
the  same  time  that  ptyalin  is  being  thus  produced  and  discharged, 
very  active  constructive  changes  are  occurring  in  the  clear  zone 
of  the  cells.  During  activity  some  at  least  of  the  mucous  cells 
seem  to  break  down,  but  it  is  probable  that  the  granules  in  the 
cell  protoplasm  become  converted  into  mucin,  which,  being 
extruded,  seem  to  destroy  the  cell  itself. 

Composition  and  Properties  of  Saliva. — While  it  is  possible 
to  draw  certain  distinctions  between  the  saliva  from  the  different 
glands,  these  distinctions  are  comparatively  unimportant,  so  far 


74  THE   PHYSIOLOGY   OF   DIGESTION  AND  ABSORPTION 

as  digestion  is  concerned;  for  the  secretions  from  the  three  pairs 
of  glands  become  mixed  in  the  mouth,  and  it  is  their  combined 
effect  which,  in  any  particular  case,  is  observed.  Saliva  con- 
tains in  1,000  parts  about  994  of  water,  the  remaining  six  parts 
being  organic  and  inorganic  solids. 

These  solids  are  chiefly  mucin,  ptyalin,  albumin  and  salts. 
The  salts  are  mainly  the  chlorides  of  sodium  and  potassium,  the 
sulphates  of  potassium,  the  phosphates  of  potassium,  sodium, 
calcium  and  magnesium,  and  sulphocyanide  of  potassium.  The 
mucin  gives  the  ropy  consistence  to  the  fluid  and  serves  a  mechan- 
ical purpose  only.  The  sulphocyanide  of  potassium  is  unusual 
in  the  body  secretions  and  its  presence  here  is  interesting.  It 
may  represent  an  end  product  of  proteid  metabolism.  The  true 
digestive  value  of  saliva  is  due  to  ptyalin,  an  amylolytic  enzyme. 

Were  it  not  for  the  presence  of  epithelial  cells  in  suspension, 
saliva  would  be  clear  and  transparent.  Its  reaction  is  alkaline, 
its  specific  gravity  is  about  1004  to  1008,  and  the  average  amount  of 
daily  secretion  is  about  2}  pounds. 

The  parotid  saliva  is  much  more  watery  and  mixes  much  more 
readily  with  the  food  than  the  submaxillary  and  sublingual, 
which  latter  is  mucilaginous  and  gives  to  the  bolus  a  glairy 
coating.  The  sublingual  saliva  is  thicker  and  more  viscid  than 
the  submaxillary. 

Nerve  Supply. — The  connection  of  the  nervous  system  with 
salivary  secretion  deserves  particular  attention,  since  the  phe- 
nomena presented  under  its  influence  are  typical,  and,  if  not 
explanatory  of  occurrences  elsewhere  in  the  body,  are  at  least 
very  suggestive. 

Each  one  of  the  three  glands  is  supplied  with  both  cerebro- 
spinal  and  sympathetic  fibers.  Each  one  of  them  has  three 
kinds  of  nerve  fibers,  secretory,  vaso-dilator  and  vaso-constrictor. 
The  secretory  and  vaso-dilator  reach  the  gland  in  the  cerebro- 
spinal  trunks;  the  vaso-constrictor  in  the  sympathetic.  The 
vaso-constrictors  and  vaso-dilators  are  distributed  to  the  walls 


SALIVARY   GLANDS  75 

of  the  blood-vessels,  and  influence  secretion  indirectly  only  by 
increasing  or  diminishing  the  amount  of  blood  going  to  the 
glands.  The  secretory  fibers  exert  their  influence  directly  upon 
the  gland  cells.  It  is  claimed  also  that  the  secretory  fibers  are 
divided  into  sets  controlling  the  production  of  the  energy-yielding 
constituents  and  sets  controlling  the  production  of  water  and  salts. 

The  parotid  gland  receives  its  cerebro-spinal  fibers  through  a 
branch  of  the  fifth  nerve,  but  when  they  are  traced  backward  it 
can  be  shown  that  they  are  in  the  tympanic  branch  of  the  ninth, 
and  pass  from  this  branch  to  the  small  superficial  petrosal  nerve 
and  thence  to  the  optic  ganglion — from  which  ganglion  they 
run  to  the  parotid  gland  by  way  of  the  auricula-temporal  branch 
of  the  third  division  of  the  fifth.  The  cervical  sympathetic  also 
sends  fibers  to  this  gland. 

The  submaxillary  and  sublingual  glands  are  supplied  by  the 
same  nerves.  Their  cerebro-spinal  fibers  leave  the  brain  by 
way  of  the  facial,  follow  the  chorda  tympani  as  far  as  a  short 
distance  beyond  its  junction  with  the  lingual  nerve,  and  then 
leave  it  to  reach  the  submaxillary  ganglion  and  run  thence  to  the 
submaxillary  and  sublingual  glands.  These  glands  receive 
sympathetic  fibers  from  the  superior  cervical  ganglion. 

Influence  of  Nerve  Supply.— Taking  the  parotid  as  an  ex- 
ample, it  is  found  that  stimulation  of  its  cerebro-spinal  fibers 
produces  an  abundant  watery  flow  of  saliva;  the  gland  becomes 
decidedly  redder,  pulsation  is  sometimes  apparent,  and  it  is 
evident  that  the  amount  of  blood  is  locally  increased.  When 
the  sympathetic  supply  of  the  parotid  is  stimulated,  the  secretion 
is  inhibited  or  reduced  to  a  minimum,  the  gland  becomes  pale 
and  the  amount  of  blood  in  it  is  evidently  diminished. 

Similar  corresponding  results  are  occasioned  in  the  submax- 
illary and  sublingual  glands  by  stimulation  of  the  chorda  tympani 
and  the  sympathetic  fibers. 

It  would  seem  at  first,  in  the  light  of  the  vascular  changes 
accompanying  stimulation  of  the  two  supplies  to  all  these  glands, 


76  THE    PHYSIOLOGY   OF    DIGESTION  AND   ABSORPTION 

that  the  resultant  phenomena  could  be  explained  entirely  by 
variations  in  the  amount  of  blood,  and  that  the  nervous  system 
influences  their  secretion  only  by  contraction  and  dilatation  of  the 
vessels.  However,  a  number  of  circumstances,  which  it  is  un- 
necessary to  relate  here,  prove  that  the  secretory  fibers  exert  an 
influence  directly  upon  the  cells  themselves,  causing  them  to 
secrete.  The  mere  distribution  of  these  fibers  to  the  gland  cells 
presupposes  some  such  function  on  their  part ;  and  it  can  actually 
be  shown  that  the  secretion  can  be  increased  when  the  blood 
supply  is  cut  'off,  or  without  dilatation  of  the  vessels.  Such 
action,  however,  is  of  course  only  temporary,  for  the  materials 
for  secretion  must  be  supplied  by  the  blood.  The  exact  method 
of  termination  of  the  secretory  fibers  has  not  been  determined. 
It  is  probable  that  they  end  between  and  around  the  cells  and  do 
not  penetrate  their  substance. 

Section  of  the  chorda  tympani  causes  a  continuous  flow  of 
saliva  from  the  submaxillary  and  sublingual  glands  for  several 
weeks.  This  has  been  termed  paralytic  secretion,  and  is  sup- 
posed to  be  due  to  the  fact  that  the  chorda  fibers  do  not  them- 
selves run  directly  to  the  glands,  but  are  distributed  to  sym- 
pathetic ganglia  (the  submaxillary  or  others  in  the  gland  sub- 
stance). Section  of  the  chorda,  then  causes,  degeneration  of 
its  fibers  only  as  for  as  these  ganglia,  and  their  cells  are  thought 
to  be  subject,  in  some  obscure  way,  to  continuous  irritation 
during  the  period  for  which  the  paralytic  secretion  continues. 

Function. — The  function  of  this  secretion  is  twofold,  (a) 
mechanical  and  (b)  chemical. 

(a)  From  a  mechanical  standpoint  (i)  it  facilitates  phona- 
tion,  mastication  and  gustation  by  maintaining  a  proper  degree 
of  moisture  in  the  mouth;  (2)  its  more  watery  parts  (parotid) 
mix  with  the  food,  dissolving  part  of  it,  so  that  it  may  be  more 
easily  masticated  and  swallowed  while  its  more  viscid  parts 
(submaxillary  and  sublingual)  spread  over  the  surface  of  the 
bolus  to  aid  in  deglutition. 


SALIVARY   GLANDS  77 

(6)  From  a  chemical  standpoint,  the  function  of  the  saliva 
is  to  convert  starch  into  sugar.  It  does  this  through  the  agency 
of  its  enzyme,  ptyalin,  which  conforms  to  the  characteristics 
of  enzymes  already  noted.  Maltose  (C12H22O11+H2O)  is  the 
form  of  sugar  produced,  but  there  are  several  intermediate  sub- 
stances formed  before  maltose  finally  results.  The  starch  mole- 
cule (C6H10O5)  was  formerly  supposed  to  simply  appropriate  a 
molecule  of  water  to  form  dextrose  (grape  sugar,  glucose, 
C6H12O6),  but  it  is  now  thought  that  there  is  a  succession  of 
hydrolytic  changes  with  the  production  of  dextrin  and  maltose. 
That  is,  the  starch  molecule  appropriates  a  molecule  of  water; 
this  new  molecule  splits  into  a  certain  kind  of  dextrin  and  mal- 
tose; the  dextrin  left  itself  appropriates  water  and  splits  up  into 
another  kind  of  dextrin  and  maltose;  this  last  dextrin  goes 
through  a  similar  process  with  a  like  result,  until  finally  only 
maltose  is  produced.  Some  dextrose  may  be  produced.  It  will 
be  seen  under  gastric  digestion  that  mineral  acidity  will  also 
convert  starch  into  sugar,  but  in  this  case  the  form  of  sugar  is 
dextrose. 

The  effect  of  temperature  on  the  action  of  enzymes  has  been 
noticed.  The  optimum  for  ptyalin  is  100°  Fahrenheit.  The 
reaction  of  saliva  is  alkaline  and  its  effect  on  starch  is  stopped 
by  an  acid  medium,  since  the  enzyme  is  thereby  destroyed. 
However,  ptyalin  has  been  shown  to  act  even  a  little  better  in 
perfectly  neutral  than  in  alkaline  solutions  (Chittenden).  The 
action  of  this  substance  on  starch  is  very  much  facilitated  if 
the  starch  be  cooked;  in  fact,  its  action  on  uncooked  starch  is 
so  slow  that  probably  it  is  inconsequential  in  digestion.  Cooked 
starch  becomes  hydrated,  and  furthermore  has  its  cellulose 
capsule  removed  from  the  granulose,  both  of  which  circumstances 
make  it  much  more  susceptible  to  salivary  influences. 

However,  it  must  be  admitted  that  the  practical  effect  of  pty- 
alin in  digestion  is  not  very  considerable  in  the  mouth  mainly 
because  the  food  is  not  kept  in  the  mouth  long  enough.  How- 


78  THE   PHYSIOLOGY   OF   DIGESTION  AND  ABSORPTION 

ever,  large  quantities  of  saliva  are  swallowed  with  the  food  and 
it  continues  its  action  in  the  stomach  while  the  food  is  stored  in 
the  cardiac  end  and  only  ceases  its  activity  when  the  food  is 
thoroughly  mixed  with  the  acid  gastric  juice.  The  conversion 
of  starch  into  sugar  is  continued  and  concluded  in  the  small 
intestine. 

Deglutition. 

The  act  of  deglutition  is  commonly  divided  into  three  periods, 
depending  upon  the  part  through  which  the  food  is  passing. 
During  the  first  period  the  bolus  passes  from  the  mouth  through 
the  isthmus  of  the  fauces,  during  the  second  through  the  pharynx, 
and  during  the  third  through  the  esophagus  into  the  stomach. 
A  brief  reference  to  the  anatomy  of  these  parts  is  necessary. 

Fauces. — The  isthmus  of  the  fauces  is  the  opening  at  the  back 
of  the  mouth,  bounded  below  by  the  base  of  the  tongue,  above 
by  the  soft  palate  and  uvula,  and  laterally  by  the  pillars  of  the 
fauces,  between  which  are  the  tonsils.  The  anterior  pillars  are 
easily  seen  when  the  mouth  is  opened  widely,  and  consist  of  the 
palatoglossi  muscles  with  their  covering  mucous  membrane. 
The  posterior  pillars  approach  each  other  more  nearly  than  the 
anterior,  and  consist  of  the  palatopharyngei  muscles  and  their 
covering  mucous  membrane. 

Pharynx. — The  pharynx  extends  from  the  basilar  process 
of  the  occipital  bone  above  about  four  and  a  half  inches  down- 
ward. It  communicates  with  the  posterior  nares,  the  mouth, 
the  Eustachian  tubes,  the  larynx  and  esophagus.  The  tube  is 
made  up  of  two  coats,  an  external  muscular  and  an  internal 
mucous.  The  muscular  coat  consists  of  the  three  constrictors 
and  the  stylopharyngeus.  The  mucous  coat  is  covered  in  its 
upper  part  with  columnar  ciliated  and  in  its  lower  part  by  pave- 
ment epithelium. 

Esophagus. — The  esophagus  runs  a  course  of  about  nine 
inches  from  the  end  of  the  pharynx,  at  a  point  behind  the  cri- 


DEGLUTITION  79 

coid  cartilage,  to  the  stomach,  which  it  enters  a  little  to  the  left 
of  the  median  line.  The  coats  of  the  esophagus  are  two,  an 
external  muscular  and  an  internal  mucous.  The  external 
coat  has  its  fibers  disposed  in  two  layers,  longitudinal  and  cir- 
cular. The  circular  layer  is  internal.  In  the  upper  third  of  the 
esophagus  the  fibers  of  the  muscular  coat  are  all  striped,  but  at 
the  beginning  of  the  middle  third  they  begin  to  give  place  to 
plain  fibers,  and  these  latter  progressively  increase,  to  consti- 
tute virtually  the  whole  muscular  coat  at  the  diaphragm.  The 
internal  mucous  coat  is  lined  by  squamous  epithelium,  and, 
except  during  the  passage  of  substances  through  the  esophagus, 
is  thrown  into  longitudinal  folds.  The  outside  fibrous  tissue 
attaches  the  whole  esophagus  to  the  surrounding  tissue. 

Mechanism  of  Deglutition. — The  first  period  of  deglutition 
is  voluntary  but  automatic,  like  respiration.  The  morsel  of 
food  is  forced  toward  and  through  the  fauces  by  the  tongue,  which 
presses  from  before  backward  against  the  hard  palate,  with  the 
bolus  above  it.  That  the  tongue  is  mainly  concerned  in  this 
act  is  shown  by  inability  to  swallow  when  this  organ  is  absent, 
unless  the  food  be  pushed  far  back  into  the  mouth  by  the  finger 
or  other  means. 

The  mechanism  of  the  second  period  is  much  more  complex. 
The  food  must  pass  through  the  pharynx  into  the  esophagus, 
and  must  not  be  allowed  to  enter  any  of  the  other  openings  com- 
municating with  the  pharynx.  The  larynx  especially  is  to  be 
protected.  Since  the  air  enters  through  the  posterior  nares 
above  the  isthmus  and  must  enter  the  larynx  in  front  of  the  esoph- 
agus, it  follows  that  the  current  of  air  would  cross  the  current 
of  food  if  swallowing  and  respiration  took  place  together.  Con- 
sequently respiration  is  suspended  during  deglutition.  As  soon 
as  the  food  has  passed  the  fauces,  the  elevators  of  the  hyoid  raise 
that  bone,  and  with  it  the  larynx.  It  is  at  the  same  time  pulled 
a  little  forward,  and  since  the  pharynx  is  attached  to  the  larynx 
posteriorly,  the  former  necessarily  follows  the  movement  of  the 


8o  THE   PHYSIOLOGY   OF    DIGESTION  AND  ABSORPTION 

latter,  and  is  thus  slipped  under  the  base  of  the  tongue  and  the 
entering  bolus.  With  elevation  of  the  larynx  the  superior  con- 
strictor of  the  pharynx  contracts  upon  the  food,  and  passes  it 
quickly  to  the  grasp  of  the  middle  constrictor,  which  in  turn 
hands  it  to  the  inferior  constrictor  and  thence  to  the  esophagus. 

The  posterior  nares  are  protected  by  contraction  of  the  pos- 
terior pillars  and  the  superior  constrictor.  The  laryngeal 
opening  is  protected  by  the  epiglottis.  When  the  tongue  is 
forced  back  and  the  larynx  raised  the  natural  effect  would  be 
to  fold  the  epiglottis  down  over  the  laryngeal  opening.  At  the 
same  time  contraction  of  the  pharyngeal  muscles  draws  to- 
gether the  sides  of  the  larynx  and  aids  in  closing  the  glottis. 
Furthermore,  the  vocal  cords  fall  together  (as  they  always  lie 
except  during  inspiration — and  inspiration  is  now  suspended). 

The  third  period  passes  the  food  through  the  esophagus  into 
the  stomach  by  contraction  from  above  downward  of  successive 
portions  of  its  muscular  wall.  Contraction  of  the  longitudinal 
fibers  draws  the  mucous  membrane  above  the  bolus.  Then  the 
circular  fibers,  contracting  in  successive  segments  from  above 
downward,  force  the  bolus  before  them.  These  movements  are 
continued  until  the  food  reaches  the  stomach.  The  time  con- 
sumed in  swallowing  a  given  article  is  about  six  seconds. 

This  is  the  mechanism  which  carries  all  materials  through  the 
alimentary  canal  from  the  esophagus  to  the  anus.  It  is  called 
peristalsis,  or  vermicular  (worm-like)  action. 

Nervous  Control. — While  nearly  all  the  muscular  tissue  con- 
cerned in  deglutition  is  of  the  striated  variety,  the  whole  proc- 
ess, except  the  first,  which  is  automatic,  must  be  considered  as 
reflex.  The  mechanism  of  deglutition  is  one  of  the  best  ex- 
amples of  finely  coordinated  muscular  action  to  be  found.  The 
afferent  fibers  concerned  are  from  the  5th,  pth,  and  loth,  and  the 
superior  laryngeal  branch  of  the  last.  The  efferent  fibers  are 
from  the  5th,  yth,  pth,  loth,  and  i2th.  The  center  for  the  re- 
flex is  supposed  to  be  far  forward  in  the  medulla. 


DIGESTION  AND  ABSOEPTION   IN  THE   STOMACH  8 1 

It  ought  to  be  added  that  the  Kronecker-Meltzer  theory  of 
deglutition  assails  with  considerable  plausibility  the  mech- 
anism of  deglutition  as  above  given.  In  a  word,  this  theory  holds 
that  when  the  bolus  of  food  rests  upon  the  dorsum  of  the  tongue, 
and  the  tip  of  that  organ  prevents,  by  its  apposition  to  the  hard 
palate,  the  escape  of  the  food  forward,  the  mylohyoids  contract 
with  great  force,  compress  the  food,  and  it  escapes  by  the  route 
of  least  resistance,  which  is  backward.  It  is  thus  shot  into  the 
esophagus,  and  the  contraction  of  the  pharyngeal  muscles  only 
supplements  that  of  the  mylohyoids. 

Digestion  and  Absorption  in  the  Stomach. 

Anatomy. — The  stomach  is  situated  beneath  the  diaphragm 
in  the  upper  part  of  the  abdominal  cavity,  and  is  moored  by  the 
esophagus  and  folds  of  the  peritoneum.  Its  general  shape  has 
been  compared  to  that  of  the  bagpipe.  Its  large,  or  fundic,  end 
is  to  the  left;  its  small,  or  pyloric,  to  the  right.  By  far  the 
greater  part  of  the  organ  is  to  the  left  of  the  median  line.  A  very 
considerable  portion  is  to  the  left  of  the  esophageal  opening. 
Except  when  distended,  its  anterior  and  posterior  walls  hang  in 
an  approximately  vertical  direction,  and  are  usually  in  contact 
by  their  mucous  surfaces.  Its  greatest  length  when  moderately 
distended  is  about  fourteen  inches,  its  transverse  diameter  about 
five  inches,  and  its  capacity  about  five  pints.  At  the  point 
where  the  anterior  and  posterior  walls  meet  inferiorly,  the  great 
omentum  (the  peritoneum  from  the  two  walls)  is  given  off.  This 
is  the  greater  curvature  and  has  the  gastro-epiploica-dextra 
and  the  gastro-epiploica-sinistra  arteries  running  along  it  between 
the  two  folds  of  the  omentum.  Where  the  anterior  and  posterior 
walls  meet  superiorly,  the  stomach  is  joined  by  the  lesser 
omentum,  the  two  layers  of  which  are  continued  in  front  and 
behind  as  the  serous  covering  of  the  stomach.  This  is  the 
lesser  curvature,  and  has  the  gastric  and  pyloric  branch  of 
6 


82  THE   PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 


FIG.  35. — Human  alimentary  canal. 

a,  esophagus;  b,  stomach;  c,  cardiac  orifice;  d,  pylorus;  e,  small  intestine;/,  biliary 
duct;  g,  pancreatic  duct;  h,  ascending  colon;  *',  transverse  colon;  /,  descending  colon'. 
k,  rectum.  (Collins  &  Rockwell.) 


DIGESTION.  AND   ABSORPTION   IN    THE    STOMACH  83 

the  hepatic  arteries  running  along  it  between  the  two  layers  of 
the  lesser  omentum.  The  large  left  hand  portion  of  the  stomach 
cavity  is  called  the  fundus  or  greater  pouch.  The  opposite 
portion  of  the  cavity  is  called  the  lesser  pouch  or  antrumpylori. 
At  one  end  is  the  cardiac  or  esophageal  opening,  at  the  other 
the  pyloric. 

Histology. — The  coats  of  the  stomach  walls  are  four.  From 
without  inward  these  are  the  (i)  peritoneal,  or  serous,  (2)  mus- 
cular, (3)  sub-mucous  and  (4)  mucous. 

1.  The  peritoneal  coat  covers  the  whole  of  the  organ  except- 
ing an  inconsiderable  linear  area,  where  the  two  layers  of  the 
lesser  (gastro-hepatic)  omentum  join  it  along  the  lesser  curvature, 
and  a  similar  area  along  the  greater  curvature,  where  the  serous 
coats  of  the  anterior  and  posterior  walls  leave  the  organ  to  form 
the  great  omentum.     This  coat  is  simply  a  fold  given  off  from 
the  peritoneum  to  envelop  the  stomach  in  practically  the  same 
manner  as  the  other  abdominal  viscera.     Its  structure  is  that  of 
serous  membranes  in  general. 

2.  The  muscular  coat,  varying  in  thickness  from  -£$  in.  over 
the  fundus  to  ^  in.  at  the  pylorus,  is  disposed  in  three  layers, 
(a)  external  longitudinal,  (b)  middle  circular  and  (c)  internal 
oblique.     The  longitudinal  fibers  are  continued  from  the  corre- 
sponding fibers  of  the  esophagus.     They  are  marked  along  the 
lesser  curvature,  but  not  very  distinct  over  other  parts.     The 
circular  fibers  are  not  abundant  to  the  left  of  the  esophageal 
opening.     They  progressively  increase  toward  the  right,  and  at 
the  pyloric  opening  constitute  a  distinct  and  powerful  muscular 
ring,   the  pyloric  sphincter,  which,   projecting  into  the  lumen 
presents  a  more  or  less  flat  surface  on  the  duodenal  side  to  pre- 
vent the  regurgitation  of  food.     The  oblique  fibers  are  supposed 
to  be  continuous  with  the  circular  fibers  of  the  esophagus.     They 
extend  over  the  greater  pouch  from  a  point  just  to  the  left  of 
the  esophageal  opening  to  a  point  on  the  greater  curvature,  about 
the  junction  of  the  middle  and  pyloric  thirds.     Here,  at  the  right 


84  THE   PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

hand  limit  of  the  oblique  fibers,  the  stomach  is  said  during  diges- 
tion to  be  considerably  constricted,  so  that  a  temporary  sphincter 
is  established.  This  point  is  the  point  of  separation  between 
the  fundus  and  the  antrum  pylori,  and  is  sometimes  called  the 
sphincter  antri  pylorici.  The  fibers  of  the  muscular  coat  are  of 
the  plain  variety,  as  is  all  the  gastro-intestinal  muscular  tissue 


Serosa.     — 


FIG.  36. — V.  S.  Wall  of  human  stomach. 
E,  epithelium;  G,  glands;  Mm,  muscularis  mucosae.      X  15.      (Stirling.) 

from  the  lower  end  of  the  esophagus  to  the  external  sphincter. 

3.  The  submucous  coat  consists  of  loose  fibre-elastic  con- 
nective tissue  which  allows  free  movement  between  the  muscular 
and  mucous  coats.     It  contains  rather  large  blood-vessels  and 
a  nerve-plexus,  the  plexus  of  Meissner. 

4.  The  mucous  coat  has  an  average  thickness  of  about  ^ 


GASTRIC    GLANDS  85 

in.,  is  loosely  attached  to  the  submucous  coat,  and,  except 
during  gastric  digestion,  is  thrown  into  longitudinal  rugae.  It 
consists  of  columnar  epithelium  resting  upon  a  basement  mem- 
brane, beyond  (underneath)  which  is  the  capillary  blood  supply. 
Throughout  the  greater  part  of  the  stomach  the  mucous  mem- 
brane can  be  shown  to  be  divided  by  delicate  connective  tissue 
into  numerous  polygonal  depressions,  from  the  bottom  of  which 
extend  the  gastric  glands. 

The  Gastric  Glands. 

In  the  mucous  membrane  of  the  stomach  are  found  two  kinds 
of  glands.  According  to  their  relative  position  with  reference 
to  the  two  ends  of  the  stomach  they  are  called  fundic  and  pyloric. 
It  is  to  be  noted,  however,  that  neither  of  these  divisions  is 
confined  strictly  to  that  portion  of  the  stomach  which  its  name 
would  seem  to  indicate.  According  to  their  secretion  the  glands 
are  called  acid  and  peptic.  The  fundic  and  acid,  and  the 
pyloric  and  peptic  are  considered  to  be  identical.  But  attention 
is  called  to  the  fact  that  while  peptic  (pyloric)  glands  secrete 
pepsin  only,  the  acid  (fundic)  secrete  both  acid  and. pepsin. 

Structure. — Some  of  the  gastric  glands  are  simple  tubules, 
while  others  may  be  bifurcated,  so  that  two  (or  more)  tubules 
communicate  with  the  surface  by  a  single  canal.  They  may  all, 
however,  be  classified  as  belonging  to  the  simple  tubular  variety. 
They  have  a  deep  secreting  portion  and  a  superficial  non-secret- 
ing portion.  The  latter  is  lined  by  columnar  epithelium,  and  is 
the  duct  proper.  The  former  is  lined  by  cuboidal  epithelium 
which  discharges  its  secretion  into  the  lumen,  this  lumen  being 
only  a  continuation  of  the  duct.  These  cuboidal  cells  are  called 
peptic  cells  because  they  produce  pepsin,  or  its  forerunner, 
pepsinogen.  The  fundic  (acid)  glands  are  found  to  have  lying 
close  to  the  basement  membrane  a  number  of  large  cells  at  inter- 
vals between  the  cuboidal  cells  and  not  extending  outward  to  the 


86 


THE   PHYSIOLOGY    OF   DIGESTION  AND   ABSORPTION 


central  lumen.  They  are  thought  to  communicate  with  the 
lumen  by  capillary  ducts,  which  may  even  penetrate  their 
substance.  They  are  supposed  to  secrete  hydrochloric  acid, 


FIG.  37. — Vertical  section  of  the  gastric  mucous  membrane. 

g,  g,  pits  on  the  surface;  p,  neck  of  a  fundus-gland  opening  into  a  duct,  g;  x,  pari- 
etal, and  y,  chief  cells;  a,  v,  c,  artery,  vein,  capillaries;  d,  d,  lymphatics,  emptying 
into  a  large  trunk,  e.  (Landois.) 

and  are  called  acid  cells  from  this  fact,  or  parietal  cells  from 
their  position.     (See  Fig.  37.) 
Method  of  Secretion. — When  food  is  ingested  gastric  move- 


GASTRIC   GLANDS 


ments  very  soon  begin,  carrying  the  food  in  this  direction  or  that, 
as  described  later.  At  the  same  time,  the  gastric  mucous  mem- 
brane changes  from  a  pale  pink  to  a  congested  red,  and  soon 
drops  of  gastric  juice  begin  to  appear.  They  run  to  the  depend- 


P"iG.  38. — Section  of  the  pyloric  mucous  membrane.     (Landois.) 

ent  portions  of  the  cavity  and  become  incorporated  with  the 
alimentary  mass.  It  is  believed  that  if  the  gastric  movements 
did  not  occur,  this  secretion  would  be  limited  for  fifteen  or  thirty 


88  THE   PHYSIOLOGY    OF    DIGESTION  AND  ABSORPTION 

minutes  to  a  very  small  area,  namely,  that  with  which  the  food  is 
in  contact.  But  it  is  comparatively  general  because  the  move- 
ments bring  practically  all  parts,  at  least  of  the  fundic  mucous 
membrane,  in  contact  with  the  food  before  this  time  has  elapsed. 
The  idea  is  that  up  to  fifteen  or  thirty  minutes  after  the  introduc- 
tion of  food,  the  glands  are  made  to  secrete  by  direct  mechanical 
stimulation  of  the  food,  and  after  this  time  the  secretion  becomes 
general,  whether  mechanical  irritation  becomes  general  or  not. 

It  ought  to  be  added,  however,  that  in  recent  years  secretion 
by  mechanical  stimulation  has  been  denied,  and  the  denial  is 
supported  by  good  evidence.  Besides  direct  proof  by  experi- 
ments, it  is  shown  that  this  early  secretion  occurs  without 
mechanical  irritation,  as  when  food  is  chewed  and  made  to  pass 
through  an  esophageal  fistula,  or  even  by  the  sight  of  food. 
These  observers  (Pawlow)  state  that  food  introduced  into  the 
stomach  through  a  fistula  produces  absolutely  no  flow  if  the 
animal  experimented  upon  does  not  know  of  the  introduction. 
Under  this  view  the  secretion  is  a  distinct  reflex,  the  impressions 
being  carried  to  the  center  by  afferent  nerves  distributed  to  the 
mouth,  or  by  nerves  of  special  sense. 

Whether  as  a  reflex  or  as  a  result  of  mechanical  stimulation, 
the  fact  remains  undisputed  that  the  flow  begins  a  few  minutes 
after  the  introduction  of  food,  and  lasts  until  gastric  digestion  is 
completed.  After  a  time  it  is  supposed  that  chemical  changes 
in  the  food  itself  further  stimulate  the  gastric  glands,  through 
their  influence  on  the  secretory  nerves.  These  stimulating 
chemical  products  are  not  developed  alike  from  all  foods;  and 
the  conclusion  is  warranted  that  some  substances  do  not  undergo 
gastric  digestion  so  readily  as  others.  Ordinary  bread  and  the 
whites  of  eggs,  for  example,  are  said  not  to  develop  them.  It 
has  been  further  shown  that  fats,  oils,  etc.,  actually  develop 
substances  which  chemically  inhibit  gastric  secretion.  There 
appears  also  to  be  a  kind  of  chemical  regulation  of  the  amount 


GASTRIC    GLANDS  89 

and  quality  of  juice,  according  as  much  or  little,  or  a  varying 
acidity,  is  needed  in  the  digestion  of  the  substance  in  the 
stomach. 

Conditions  influencing  digestion  operate  mainly  by  producing 
changes  in  the  quantity  or  quality  of  gastric  juice,  and  these 
changes  in  turn  are  largely  effected  through  the  nervous  system. 
Fever,  overeating,  depressing  emotions,  strenuous  physical  or 
mental  exercise,  etc.,  decrease  the  secretion  and  correspond- 
ingly interfere  with  digestion. 

Changes  During  Activity.— Like  the  salivary  cells,  the  cu- 
boidal  peptic  cells  can  be  shown  to  undergo  changes  during 
secretory  activity.  When  at  rest  they  contain  abundant  gran- 
ules, but  during  secretion  these  granules  disappear,  first  from  the 
base  and  later  from  well-nigh  the  whole  cell.  The  granules  are 
supposed  to  contain  pepsin,  or  rather  pepsinogen,  for  it  is  thought 
that  pepsin  is  not  formed  by  the  cell  directly,  but  is  made  out  of 
pepsinogen,  which  is  the  product  of  the  peptic  cells,  probably 
under  the  action  of  hydrochloric  acid.  The  rennin  is  also  sup- 
posed to  exist  in  the  cells  as  some  preliminary  material  cor- 
responding to  pepsinogen.  This  material  may  be  termed 
rennin  zymogen. 

Changes  in  the  acid  cells  during  activity  also  occur,  but  are 
more  obscure  than  those  in  the  peptic  cells.  The  source  of 
hydrochloric  acid  is  a  decomposition  of  the  neutral  chlorides  of 
the  blood  and  the  union  of  the  chlorine  thus  liberated  with  hydro- 
gen, but  how  or  why  this  occurs  is  not  explained  by  phenomena 
so  far  observed. 

Secretory  Nerves. — While  it  has  been  impossible  to  demon- 
strate secretory  fibers  to  the  cells  of  the  gastric  glands,  such  fibers 
must  exist  in  the  vagus.  Section  of  it  (and  the  sympathetic), 
however,  does  not  entirely  stop  the  secretion,  but  incidents  re- 
ferred to  in  a  preceding  section,  such  as  secretion  at  sight 
of  food,  or  when  food  is  chewed  and  not  swallowed,  certainly 


90  THE    PHYSIOLOGY   OF    DIGESTION  AND   ABSORPTION 

point  to  an  influence  of  the  central  system  over  secretion.  Of 
course  the  sympathetic  fibers  to  the  vessel  walls  are  indirectly 
concerned. 

Condition  of  Food  on  Entering  Stomach.— The  food 
enters  the  stomach  in  the  same  condition  in  which  it  left  the 
mouth.  It  has  been  more  or  less  completely  triturated  by  mas- 
tication; the  whole  has  been  moistened,  and  a  part  dissolved 
by  the  saliva.  All  the  materials  taken  in  have  been  thoroughly 
mixed  with  each  other,  and  some  of  the  starch  has  been  con- 
verted into  sugar.  The  reaction  is  now  alkaline,  unless  the  acid- 
ity of  the  articles  taken  has  been  too  great  to  be  overcome  by 
the  alkalinity  of  the  saliva — in  which  case  there  would  be  no 
amylolytic  change.  Excepting  starch,  all  foods  entering  the 
stomach  are  chemically  unaffected.  It  remains  to  see  what  hap- 
pens to  the  foods  under  the  influence  of  gastric  digestion.  These 
changes  are  brought  about  by  the  gastric  juice  aided  by  muscular 
movements  of  the  stomach. 

Properties  and  Composition  of  Gastric  Juice. — The  secre- 
tion of  the  glands  of  the  stomach  is  called  gastric  juice.  Gas- 
tric juice  may  be  secured  in  several  ways,  but  the  most  reliable 
article  for  experimentation  is  taken  from  a  previously  established 
gastric  fistula  in  one  of  the  lower  animals.  It  is  a  thin,  almost 
colorless  liquid  of  an  acid  reaction,  and  a  specific  gravity  of  1005 
to  1009.  Chemically  it  contains  per  thousand  about  973  parts 
water  and  27  solids.  Proteid  substances  compose  some  17  of  the 
27  parts  of  solid  matter.  These  substances  are  mainly  mucin, 
pepsin  and  rennin.  The  most  important  non-nitrogenous 
constituent  is  free  hydrochloric  acid.  The  others  are  chiefly 
the  chlorides  of  sodium,  potassium,  calcium,  and  ammonium, 
and  the  phosphates  of  iron,  calcium,  and  magnesium.  The 
amount  of  gastric  juice  secreted  in  twenty-four  hours  is  from  six 
to  fourteen  pounds.  Gastric  juice  will  resist  putrefaction  for  a 
long  time,  probably  on  account  of  the  free  acid.  Its  digestive 


GASTRIC    GLANDS  9! 

properties  are  due  to  the  proteolytic  enzyme,  pepsin,  the  milk- 
curdling  enzyme  rennin,  and  the  free  hydrochloric  acid. 

Hydrochloric  Acid. — The  amount  of  free  hydrochloric  acid 
present  in  normal  gastric  juice  is  from  two-tenths  to  three-tenths 
of  one  per  cent.  It  has  been  frequently  claimed  that  the  acidity 
of  this  secretion  is  due  to  lactic  acid,  but  while  it  cannot  be 
denied  that  lactic  acid,  from  the  fermentation  of  carbonydrates 
is,  or  may  be,  normally  in  the  stomach  during  digestion,  yet 
hydrochloric  acid  is  undoubtedly  the  free  acid  proper  to  the  gas- 
tric juice.  Digestion,  however,  will  proceed  under  a  proper 
(variable)  degree  of  an  acidity  from  almost  any  acid. 

Beyond  an  insignificant  effect  in  converting  cane  sugar  into 
dextrose,  its  function  is  a  passive  one,  namely,  that  of  simply 
making  the  secretion  acid,  so  that  pepsin  may  act  upon  the 
proteids. 

Pepsin. — Pepsin  is  a  proteolytic  enzyme,  the  composition  of 
which  has  not  been  determined.  From  the  definition,  it  con- 
verts proteids  into  peptones.  It  operates  only  in  an  acid  me- 
dium. Hence  its  action  is  contingent  upon  the  presence  of  an- 
other constituent  of  the  gastric  juice,  namely,  hydrochloric  acid. 
Pepsin  is  a  typical  enzyme,  and  reference  to  the  characteristics 
of  those  bodies  will  avoid  repetition  of  its  properties  here. 

Rennin. — Rennin  has  the  property  of  coagulating  milk.  It 
acts  upon  the  soluble  proteid  of  milk  (casein),  changing  it  into 
an  insoluble  product,  which  is  precipitated.  Acids  also  will 
coagulate  casein.  Milk  when  left  standing  at  ordinary  tem- 
perature has  lactic  acid  produced  by  the  action  of  bacteria  upon 
the  lactose  in  it,  and  this  acid  precipitates  the  curd.  The  acid 
of  the  gastric  juice  might  be  sufficient  to  bring  about  this  result, 
but  the  quick  coagulation  of  milk  when  it  is  introduced  into 
the  stomach  is  probably  not  due  to  the  acid,  since  neutral  ex- 
tracts of  the  gastric  mucous  membrane  will  themselves  curdle 
milk.  After  coagulation  the  action  of  pepsin  begins  and  the 


92  THE   PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

casein  is  converted  into  peptones  in  the  usual  manner.     The 
value  of  the  curdling  process  is  not  apparent. 

Action  of  Gastric  Juice  on  Foods.  (A)  On  Proteids.—A 
familiar  test  for  the  proper  performance  of  gastric  digestion  is 
the  observation  of  the  effect  of  the  juice  in  a  given  case  upon 
the  white  of  an  egg  (proteid).  In  normal  gastric  juice,  or  in  a 
properly  prepared  artificial  solution,  the  egg  is  seen  to  swell  up 
and  dissolve.  This  soluble  proteid  is  now  called  peptone,  and 
it  differs  from  the  proteid  of  the  egg  in  certain  important  re- 
spects, to  be  noted  later.  But,  although  peptone  is  the  final 
product  of  pepsin-hydrochloric  action,  there  are  certain  sub- 
stances produced  intermediate  between  the  initial  proteid  and  the 
final  peptone,  just  as  in  case  of  the  formation  of  maltose  by 
ptyalin.  Some  of  these  substances  have  been  called  acid-albu- 
min, parapeptone,  propeptone,  etc.  But  whatever  they  may  be, 
the  nomenclature  of  Kuhne  is  being  largely  followed  at  present. 
He  supposes  that  the  first  product  is  an  acid  albumin  which  he 
calls  syntonin;  that  syntonin  under  the  influence  of  pepsin 
undergoes  hydrolysis,  taking  up  water  and  splitting  into  primary 
proteoses ;  that  each  of  these  primary  proteoses  takes  up  water 
and  splits  into  secondary  proteoses ;  that  these  last  undergo  a 
similar  change  with  the  production  of  peptones ;  so  that  the  suc- 
cessive substances  are  proteid,  syntonin  (acid-albumin),  primary 
proteoses,  secondary  proteoses,  peptones. 

Peptones  can  be  shown  to  be  different  from  syntonin  and 
the  proteoses  by  chemical  reaction.  The  chief  object  of  proteo- 
lytic  digestion  is  to  get  the  proteids  into  a  diffusible  condition. 
Peptones  differ  from  proteids  in  at  least  three  important  respects: 
(i)  They  can  pass  through  animal  membranes,  that  is,  can  be 
absorbed;  (2)  they  are  no  longer  coagulable  by  heat  or  many  acids; 
(3)  they  are  capable^  of  assimilation  by  the  cells  after  they  have 
been  absorbed. 

(B)  On  Carbohydrates. — There  is  no  enzyme  furnished  by 
the  stomach  to  affect  any  of  the  carbohydrates.  It  is  true  that 


GASTRIC   GLANDS  93 

salivary  digestion  proceeds  in  some  small  degree  in  the  stomach. 
Saliva  is  swallowed  with  the  food,  and  until  the  reaction  becomes 
acid  (which  cannot  be  immediately),  there  is  no  reason  why  the 
conversion  of  starch  into  maltose  should  not  proceed.  It  is  also 
true  that  the  mere  acid  of  the  gastric  juice  can  slowly  convert 
cane  sugar  into  dextrose.  Simple  acidulated  water  will  do  the 
same. 

(C)  On  Fats. — Neither  is  there  any  fat-splitting  enzyme  in  the 
gastric  secretion.     So  far  as  any  chemical  change  is  concerned 
the  fats  leave  the  pylorus  in  exactly  the  same  condition  as  they 
entered  the  mouth.     Their  physical  condition,  however,  under- 
goes some  change  in  the  stomach.     The  body  temperature  is 
sufficient  to  liquefy  them,  the  vesicles  in  which  the  droplets  are 
contained  are  dissolved,  and  thus  set  free,  they  become  a  part 
of  the  mechanical  mixture,  chyme,  and  are  made  easier  subjects 
of  intestinal  digestion. 

(D)  On  Albuminoids. — The  albuminoids  are  acted  upon  by 
pepsin  and  hydrochloric  acid  in  much  the  same  way  as  are  the 
proteids.     Taking  gelatin  as  a  type,  gelatoses  are  formed  instead 
of  proteoses.     It  is  stated  that  peptic  digestion  does  not  go  fur- 
ther than  the  gelatose  stage  with  the  albuminoids,  conversion 
into  peptones  taking  place  under  the  influence  of  trypsin. 

Resistance  of  Stomach  Wall  to  Digestion. — It  would  be  in- 
teresting to  know  why  the  stomach  (or  the  intestine)  does  not 
digest  itself.  If  a  portion  of  the  stomach  of  another  animal  be 
placed  in  that  of  a  living  animal,  it  will  be  digested;  or  if  the 
circulation  be  cut  off  from  a  limited  area  of  the  stomach,  the 
secretion  will  frequently  digest  that  part  of  the  organ  and  bring 
about  a  perforation;  or  further,  if  any  living  part  of  an  animal, 
as  the  leg  of  a  frog,  be  fastened  in  the  stomach  of  another  animal, 
it  will  likewise  be  digested.  The  last  instance  would  seem  to 
lead  to  the  conclusion  that  living  matter  can  be  digested,  but 
in  reality  it  is  shown  (Bernard)  that  the  tissue  is  first  killed  by 
the  acid,  and  that  no  digestion  takes  place  in  the  alkaline  intes- 


94  THE   PHYSIOLOGY    OF    DIGESTION   AND   ABSORPTION 

tinal  juice.  But  why  the  stomach  is  not  thus  attacked  when 
other  living  tissue  is  remains  obscure.  The  most  plausible 
theory  is  that  the  gastric  epithelium  is  possessed  of  some  power, 
mechanical  or  physical,  the  nature  of  which  is  unknown,  inhibit- 
ing the  action  of  the  gastric  juice,  most  probably  by  preventing 
its  absorption. 

"A  nearer  approach  to  an  explanation  seems  to  have  been 
attained  in  the  discovery  of  an  antipeptic  and  antitryptic  action 
of  the  stomach  and  intestinal  mucosa.  This  action  is  probably 
due  to  antienzymes  which  are  found  throughout  the  whole  animal 
scale  and  occur  not  only  in  the  intestinal  tract,  but  also  in  cells 
of  other  organs.''  (Tigerstadt.) 

Movements  of  the  Stomach. — Whether  the  exact  details  of 
the  muscular  movements  of  the  stomach  be  known  or  not,  the 
essential  fact  to  be  remembered  is  that  the  organ  is  in  a  more  or 
less  continuous  state  of  muscular  activity  for  several  hours  after 
the  ingestion  of  an  ordinary  meal,  and  that  this  activity  results  in 
the  physical  disintegration  of  most  of  the  solids  introduced,  in  the 
thorough  mixing  of  all  the  classes  of  foods  with  each  other  and 
with  the  gastric  juice,  and  in  the  passage  from  time  to  time  of 
such  parts  as  have  been  reduced  to  a  pultaceous  condition 
through  the  pylorus  into  the  duodenum,  until  finally  the  stomach 
is  empty. 

In  considering  the  mechanism  of  these  movements  a  division 
of  the  organ  into  two  segments,  f undic  and  pyloric,  by  the  sphinc- 
ter antri  pylorici  is  to  be  kept  in  mind.  When  food  has  entered 
the  stomach  the  peristaltic  wave  of  contraction  begins  at  the 
splenic  end  and  passes  toward  the  right.  This  contraction 
is  comparatively  weak,  is  mainly  evident  along  the  greater 
curvature,  and  increases  in  strength  as  it  passes  toward  the 
pylorus.  Its  wave-like  character  is  due  to  the  contraction  and 
subsequent  relaxation  of  successive  bands  of  circular  and  oblique 
fibers.  Regurgitation  of  food  is  prevented  by  a  rhythmical 
contraction  of  the  lower  end  of  the  esophagus,  and  the  effect 


GASTRIC    GLANDS  95 

of  this  muscular  wave  (peristalsis)  in  the  fundus  is  to  force  the 
food  toward  the  pylorus.  But  when  the  right  end  is  reached, 
the  rather  firm  contraction  of  the  sphincter  antri  pylorici  pre- 
vents the  entrance  into  the  antrum  of  all  except  the  liquid  or 
semi-liquid  parts.  The  food,  thus  denied  admission  to  the 
antrum,  takes  a  course  along  the  lesser  curvature  to  the  splenic 
end,  then  back  along  the  greater  curvature,  and  such  parts  of 
it  as  have,  during  this  revolution,  been  sufficiently  dissolved 
pass  into  the  antrum.  These  revolutions  continue  until  the 
fundus  has  been  emptied. 

It  is  not  to  be  supposed  that  food  has  been  accumulating 
meantime  in  the  antrum.  Indeed,  it  is  certain  that  muscular  con- 
tractions are  here  much  more  active  than  in  the  fundus,  where 
the  movements  are  slow  and  of  a  rather  compressing  nature. 
It  is  thought  that  very  soon  after  the  entrance  of  food  from  the 
fundus  the  entire  muscular  wall  of  the  antrum  undergoes  very 
strong  contraction  of  a  peristaltic  nature,  and  the  pultace- 
ous  parts  of  its  contents  are  sent  with  some  force  into 
the  duodenum.  Those  which  are  not  sufficiently  dissolved  to 
pass  the  pyloric  sphincter  are  said  to  excite  an  anti-peristaltic 
movement,  whereby  they  are  thrown  back  into  the  fundus  for 
further  digestion — the  sphincter  antri  pylorici  having  now  relaxed. 
However,  substances  which  the  gastric  juice  and  contractions 
cannot  dissolve  will  finally  pass  the  pylorus,  but  they  are  prob- 
ably delayed  for  a  considerable  time. 

This  succession  of  movements  is  continued  with  a  rapidity 
and  regularity  varying  with  the  condition  of  the  organ  and  the 
nature  of  its  contents.  They  last  until  the  organ  is  emptied 
in  part  by  the  absorption  of  its  contents,  but  mainly  by  their 
passage  into  the  small  intestine.  Each  circuit  in  the  fundus 
probably  occupies  about  three  minutes,  and  gastric  digestion 
as  a  whole  lasts  usually  from  two  to  five  hours.  The  contrac- 
tion and  relaxation  of  plain  muscle  is  much  slower  than  that  of 
striped. 


96  THE  PHYSIOLOGY  OF   DIGESTION  AND  ABSORPTION 

It  is  the  fundus,  and  not  the  pylorus,  which  serves  as  a  reservoir 
and  in  which  the  greater  part  of  gastric  digestion  occurs.  The 
precise  condition  of  the  pyloric  sphincter  during  gastric  digestion 
is  unknown.  It  may  have  simply  an  exalted  degree  of  tonicity 
which  does  not  completely  close  the  opening  and  which  can  be 
overcome  by  pressure,  or  it  may  be  tightly  contracted  and  require 
a  distinct  nervous  dispensation  to  effect  its  relaxation  for  the 
passage  of  fluids  as  well  as  solids.  It  would  seem  that  the  length 
of  time  for  which  food  is  detained  in  the  stomach  depends  more 
upon  its  physical  condition  than  upon  its  chemical — that  is, 
that  upon  any  stage  of  digestion  which  it  may  have  reached; 
for  it  can  be  shown  that  fluids  pass  very  quickly  into  the 
intestine. 

The  secretory  occurrences  during  these  movements  are  of  the 
greatest  importance  (see  p.  86-88). 

Nerve  Supply. — The  stomach  is  supplied  with  pneumogastric 
and  sympathetic  fibers.  The  latter  can  be  traced  through  the 
solar  plexus,  splanchnics  and  cervical  ganglia  to  the  spinal  cord. 
They  exert  an  inhibitory  effect  on  the  muscular  tissues;  their 
stimulation  causes  relaxation.  The  vagus  fibers  are  motor; 
their  stimulation  causes  contraction.  But  these  nerves  serve  only 
to  regulate  the  muscular  movements.  It  is  the  stimulus  of  food  in 
the  stomach  which  excites  gastric  peristalsis.  It  is  not  stopped 
by  section  of  these  nerves,  though  it  may  be  interfered  with. 
This  stimulation  is  exerted  either  directly  upon  the  nerve  fibers 
or  upon  the  ganglia  of  the' stomach  wall. 

The  conditions  influencing  gastric  digestion  operate  mainly 
through  changes  in  the  quality  and  quantity  of  gastric  juice. 

Digestion  and  Absorption  in  the  Intestines. 
The  Small  Intestine. 

Anatomy. — The  small  intestine  extends  from  the  pylorus  to 
the  caput  coli,  and  is  about  twenty  feet  in  length.  It  lies  in 


DIGESTION  AND   ABSORPTION   IN   THE    INTESTINES  97 

numerous  coils  which  are  held  loosely  in  place  by  a  fold  of  perito- 
neum running  from  one  side  of  the  great  abdominal  vessels, 
enveloping  the  gut,  and  returning  to  the  parietal  wall  on  the 
opposite  side  of  the  vessels.  The  fold  thus  attaching  the  intestine 
to  the  abdominal  wall  is  the  mesentery.  The  distance  along 
the  mesentery  from  this  parietal  region  to  the  gut  is  three  or 
four  inches,  except  at  the  beginning  and  end  of  the  small  intes- 
tine, where  it  is  shorter,  to  bind  the  tube  more  firmly  in  place. 
The  upper  eight  or  ten  inches  of  the  small  gut  is  called  the 
duodenum,  the  next  eight  feet  the  jejunum,  and  the  remainder 
the  ileum.  No  anatomical  peculiarity  separates  these  parts. 
Their  average  diameter  is  about  one  and  a  quarter  inches. 

Histology. — The  wall  of  the  intestine  is  in  four  layers, 
serous,  muscular,  submucous  and  mucous.  The  serous  layer 
consists  of  the  enveloping  fold  of  peritoneum  and  needs  no 
description,  except  that,  like  serous  membranes  elsewhere,  it 
furnishes  a  lubricating  secretion  to  provide  for  the  easy  gliding 
of  the  intestines  over  each  other  and  over  the  other  viscera.  The 
muscular  coat  has  its  muscular  fibers  disposed  in  two  layers, 
an  external  longitudinal  and  an  internal  circular.  The  latter 
is  the  stronger.  Between  the  two  muscular  layers  is  the  nervous 
plexus  of  Auerbach.  Between  the  circular  layer  and  the  mucous 
coat  is  the  submucous  layer  which  contains  the  nerve  plexus  of 
Meissner.  These  communicate  with  others  by  fibers  of  extension. 
The  mucous  coat  presents  several  points  deserving  mention. 
These  are  (i)  valvulae  conniventes;  (2)  villi;  (3)  secreting  glands, 
(a)  of  Brunner  and  (b)  of  Lieberkuhn;  (4)  solitary  and  agminate 
glands. 

i.  The  valvulae  conniventes  are  simply  transverse  folds  or 
tucks  of  the  entire  mucous  membrane,  each  of  which  extends 
from  one-third  to  one-half  around  the  circumference  of  the  tube 
and  projects  by  its  middle  portion  sometimes  to  the  center  of  the 
lumen.  These  small  folds,  800  to  1,000  in  number,  extend  from 
about  the  middle  of  the  duodenum  to  the  beginning  of  the  last 
7 


98  THE   PHYSIOLOGY   OF   DIGESTION  AND  ABSORPTION 

third  of  the  ileum  and  greatly  increase  the  length  of  the  mucous 
membrane  over  that  of  the  gut  proper.  They  are  not  effaced 
by  the  passage  of  food  or  by  other  circumstances,  for  the  two 
surfaces  of  the  fold  which  are  in  apposition  are  bound  together 
by  loose  connective  tissue.  The  fold  as  a  whole,  however,  is 
freely  movable  upward  or  downward  in  the  intestine  and  has  no 
tendency  to  obstruct  the  canal.  The  only  function  of  the  val- 
vulae  conniventes  is  to  furnish  a  greater  secreting  surface  and, 


FIG.  39. — Diagram  of  a  longitudinal  section  of  the  wall  of  the  small  intestine. 

a,  villi;  b,  Lieberkuhn's  glands;  c,  tunica  muscularis  mucosse,  below  which  lies 
Meissner's  nerve  plexus;  d,  connective  tissue  in  which  many  blood  and  lymph  vessels 
lie;  e,  circular  muscle  fibers  cut  across  with  Auerbach's  nerve  plexus,  below  it;/, 
longitudinal  muscle  fibers;  g,  serous  coat.  (Yeo.) 

by  somewhat  retarding  the  passage  of  the  alimentary  mass,  to 
subject  it  for  a  longer  time  to  the  digestive  fluids. 

2.  The  villi  are  especially  important  in  connection  with 
absorption,  and  their  description  properly  belongs  under  that 
head.  They  are  conical  elevations  responsible  for  the  velvety 
character  of  the  mucous  membrane.  They  exist  in  great  num- 
bers from  the  pylorus  to  the  ileo-cecal  valve,  covering  the  valvulae 
conniventes  as  well  as  the  general  surface  of  the  mucous  mem- 
brane. The  largest  are  about  -fa  in.  long  and  T^  m-  m  diameter 
at  their  base.  They  are  only  elevations  of  the  mucous  membrane 
containing  a  central  tube,  the  lacteal,  which  is  nothing  but  an 


DIGESTION  AND   ABSORPTION   IN   THE    INTESTINES 


99 


intestinal  lymphatic.  The  structure  from  without  inward — 
that  is,  from  the  surface  of  the  villus  inward  to  its  center — is  (i) 
a  layer  of  columnar  epithelium  resting  upon  a  delicate  basement 


FIG.  40. — Portion  of  the  wall  of  the  small  intestine  laid  open  to  show  the 
valvulae  conniventes.     (From  Yeo  after  Brinton.} 

membrane;  (2)  lymphoid  tissue  containing  abundant  capillaries 
and  connective  tissue  cells;  (3)  a  thin  layer  of  plain  muscle  fibers 
continuous  from  the  intestinal  wall;  (4)  the  lacteal,  whose  endo- 
thelial  wall  contains  many  stomata. 


FIG.  41. — Vertical  section  of  a  villus  of  the  small  intestines  of  a  cat. 

a,  striated  border  of  the  epithelium;  b,  columnar  epithelium;  c,  goblet  cells;  d, 
central  lymph- vessel ;  e,  smooth  muscular  fibers;/,  adenoid  stroma  of  the  villus  in 
which  lymph  corpuscles  lie.  (Kirkes  after  Klein.) 

3.  The  glands  of  Brunner  and  the  crypts  of  Lieberkuhn, 

or  intestinal  tubules,  are  supposed  to  produce  the  succus  entericus. 


100         THE   PHYSIOLOGY    OF    DIGESTION  AND  ABSORPTION 

The  former  are  chiefly  limited  to  the  upper  half  of  the  duodenum. 
The  latter  exist  throughout  the  small  and  large  intestine. 

4.  The  solitary  and  agminate  glands  are  not  supposed  to 
contribute  to  the  production  of  the  intestinal  juice.  They  are 
alike  in  structure,  the  agminate  glands  being  only  a  collection 
of  solitary  glands.  The  former  are  the  Peyer's  patches,  so 
important  in  the  pathology  of  typhoid  fever.  These  patches 
are  usually  about  twenty  in  number  and  confined  to  the  lower 
two-thirds  of  the  ileum,  where  they  occupy  that  portion  of  the 
circumference  of  the  tube  opposite  the  attachment  of  the  mes- 
entery. Their  average  dimensions  are  iXiJ  in.  They  consist 
essentially  of  lymphoid  tissue,  the  separate  follicles  of  which 
are  surrounded  by  lymphatics  and  penetrated  by  blood-vessels. 
They  are  covered  by  villi,  but  the  valvulae  conniventes  cease  at 
their  edges.  The  solitary  glands  are  more  widely  distributed 
than  the  agminate. 

The  chyme,  having  passed  from  the  stomach  to  the  small  in- 
testine, encounters  three  digestive  fluids,  pancreatic  juice,  bile 
and  intestinal  juice.  These  are,  of  course,  mixed  together,  but 
none  interferes  with  the  action  of  the  other. 

The  Pancreas. — The  pancreas  is  a  large  gland  lying  in  the 
upper  part  of  the  abdominal  cavity  behind  the  stomach.  It  has 
the  general  shape  of  a  hammer,  its  head  being  embraced  by  the 
bend  of  the  duodenum  and  its  opposite  extremity  reaching  to 
the  spleen.  It  weighs  some  four  or  five  ounces,  and  is  about 
seven  inches  long.  Its  duct,  the  duct  of  Wirsung,  usually 
joins  the  common  bile  duct  just  where  this  latter  penetrates  the 
wall  of  the  duodenum,  so  that  the  bile  and  pancreatic  juice 
enter  the  small  intestine  together.  Sometimes  the  two  ducts  do 
not  join,  and  sometimes  a  second  smaller  duct  from  the  pan- 
creas penetrates  the  duodenum  a  little  below  the  larger  one. 
The  duct  of  Wirsung  traced  backward  divides  and  subdivides 
until  its  final  ramifications  end  in  the  alveoli,  or  secreting 
portions. 


THE   PANCREAS  IOI 

Histology. — This  is  a  compound  tubular  gland.  The  cells 
in  the  alveoli  are  of  the  serous  type  and  are  granular  toward  the 
central  lumen.  During  activity  they  undergo  changes  Very 
similar  to  the  salivary  cells,  the  non-granular  zone  toward  the 
basement  membrane  increasing  and  extending  and  the  granular 
zone  becoming  correspondingly  smaller.  Here,  as  in  the  sali- 


4 

FIG.  42. — One  saccule  of  the  pancreas  of  the  rabbit  in  different  states  of 
activity.     (From  Brubaker  after  Yeo.) 

A ,  after  a  period  of  rest,  in  which  case  the  outlines  of  the  cells  are  indistinct  and  the 
inner  zone — *'.  e.,  the  part  of  the  cells  (a)  next  the  lumen  (c) — is  broad  and  filled  with 
fine  granules.  B,  after  the  gland  has  poured  out  its  secretion,  when  the  cell  outlines 
(d)  are  clearer,  the  granular  zone  (a)  is  smaller,  and  the  clear  outer  zone  is  wider. 

vary  glands,  it  is  believed  that  the  granules  are  made  from  the 
clear  protoplasm,  and  contain  the  enzymes  or  their  formative 
materials.  The  formative  material  in  all  these  glands  is  given 
the  name  of  zymogen,  although  the  zymogen  in  a  particular 
gland  may  have  a  particular  name,  as  pepsinogen,  the  forerunner 
of  pepsin,  or  trypsinogen,  the  forerunner  of  trypsin. 

Properties  and  Composition  of  Pancreatic  Juice.— The 
pancreatic  juice  is  a  colorless  liquid,  alkaline  in  reaction,  and  has 
a  specific  gravity  of  about  1040  if  taken  from  a  recent  fistula.  It 
coagulates  when  heated  and  is  prone  to  putrefaction  on  exposure. 
With  a  specific  gravity  of  about  1040,  it  contains  per  thousand 
about  900  parts  water,  the  remainder  being  different  solid  food 
materials  in  solution.  These  constituents  are  a  proteid  and 


102         THE    PHYSIOLOGY   OF    DIGESTION  AND   ABSORPTION 

three  very  important  digestive  ferments,  trypsin,  steapsin  and 
amylopsin.  -  The  phosphates  and  carbonates  are  plentiful  and 
give  the  fluid  its  alkaline  reaction. 

Trypsin. — Trypsin,  like  pepsin,  converts  proteids  into  pep- 
tones. Nothing  positive  is  known  of  its  composition,  but  it  is 
possessed  of  the  usual  characteristics  of  enzymes  regarding  tem- 
perature, etc.  It  differs  from  pepsin  in  that  its  proteolytic  action 
is  more  powerful  and  can  take  place  in  alkaline  media.  It  will 
also  act  in/neut^'aJ  or  weakly  acid  media.  The  opinion  is  ad- 
vanced that  while  ihe  gastric  juice  is  capable  of  converting  pro- 
ftjids  into  pfeptones;  as  &  matter  of  fact  it  does  not  usually  carry 
the  process  further  than  the  proteose  stage,  and  thus  prepares 
the  proteoses  for  tryptic  digestion. 

It  was  seen  that  the  successive  products  of  pepsin-hydrochloric 
digestion  are  syntonin,  primary  proteoses,  secondary  proteoses 
and  peptones.  In  tryptic  digestion  it  seems  that,  in  the  splitting 
process,  the  syntonin  (here  alkali-albumin)  and  primary  proteose 
stages  are  omitted,  and  the  first  product  is  secondary  proteoses, 
which  are  split  into  peptones.  Furthermore,  trypsin  goes  a  step 
beyond  with  some  of  the  peptones  and  converts  them  into 
simpler  compounds,  the  best  known  of  which  are  leucin  and 
tyrosin.  These  are  found  normally  in  the  intestinal  canal,  but 
the  physiological  importance  of  this  conversion  is  not  apparent. 
The  opinion  that  it  is  a  useless  sacrifice  of  useful  peptones  does 
not  seem  warranted. 

Amylopsin. — The  amylolytic  enzyme,  amylopsin,  is  identical  in 
its  action  with  ptyalin.  This  enyzme  is  very  important,  for  it  has 
been  remarked  that  the  action  of  ptyalin  is  probably  rather  incon- 
sequential, and  by  far  the  greater  portion  of  the  starch,  which 
constitutes  a  large  part  of  our  ordinary  food,  must  be  digested 
in  the  small  intestine — and  almost  entirely  by  amylopsin. 

Steapsin. — Under  the  influence  of  steapsin  neutral  fats  take 
up  water  and  undergo  hydrolysis,  with  the  production  of  glyc- 


INTERNAL  PANCREATIC  SECRETION  103 

erine  and  the  fatty  acid  corresponding  to  the  kind  of  fat  which  is 
split  up.  In  the  intestine  it  is  probable  that  only  a  part  of  the 
neutral  fats  are  thus  split  in  glycerine  and  fatty  acids.  The 
fatty  acids  thus  formed  unite  with  the  alkaline  salts  to  form  soaps, 
and  these  soaps,  aided  by  intestinal  peristalsis,  convert  the 
remaining  fats  into  an  emulsion.  The  products  of  fat  digestion 
are  therefore  glycerine,  soaps,  and  emulsions,  all  of  which  can 
be  absorbed  in  a  way  to  be  noted  later.  While  the  emulsification 
of  fats  under  the  influence  of  soaps  (fatty  acids  and  alkaline  salts) 
is  an  undoubted  effect,  the  method  of  procedure  is  unknown. 
It  is  certain  that  the  emulsification  is  aided  by  the  presence  of 
bile,  although  this  fluid  possesses  no  fat-splitting  enzyme. 

Method  of  Secretion.  It  can  be  shown  that  the  secretion 
begins  to  be  discharged  into  the  duodenum  very  soon  after  the 
entrance  of  food  into  the  stomach,  and  continues  as  long  as 
intestinal  digestion  is  in  progress.  Consequently  the  flow  will  be 
intermittent  if  the  meals  are  far  enough  apart.  It  is  almost 
certain  that  the  secretion  is  a  reflex  act  as  a  result  of  impressions 
upon  the  mucous  membrane  of  either  the  stomach  or  the  duo- 
denum. The  acidity  of  the  gastric  juice  seems  to  be  the  natural 
stimulus  and  to  exert  its  influence  upon  the  duodenal  mucous 
membrane.  This  is  not  incompatible  with  the  early  flow  after 
the  ingestion  of  food,  for  it  will  be  seen  later  that  at  least  a  small 
quantity  of  that  food  passes  quickly  to  the  duodenum  and  carries 
gastric  juice  with  it.  The  composition  of  the  secretion  seems  to 
be  influenced  in  some  degree  by  the  character  of  the  food.  It  is 
interesting  that  oils  increase  the  pancreatic  flow. 

Nerve  Supply. — The  pancreas  has,  besides  vaso-motor  fibers 
to  its  vessels,  distinct  secretory  fibers,  like  those  of  the  salivary 
glands.  These  fibers  probably  run  in  both  the  sympathetic  and 
the  vagus. 

Internal  Pancreatic  Secretion.  Circumstantial  evidence 
leaves  scarcely  any  doubt  that  the  pancreas  produces  some  sub- 
stance which  is  discharged  into  the  blood  and  markedly  influ- 


IO4         THE   PHYSIOLOGY    OF   DIGESTION  AND   ABSORPTION 

ences  nutrition.  Removal  of  the  gland  is  followed  by  death  from 
inanition  in  two  or  three  weeks;  and  previous  to  that  sequel  the 
most  striking  phenomenon  is  marked  glycosuria,  with  the  ordi- 
nary symptoms  of  diabetes  mellitus.  Retention  of  a  compara- 
tively small  portion  of  the  gland  obviates  this  condition.  Sugar 
does  not  exist  normally  in  the  blood,  and  this  internal  secretion 
may  contain  some  ferment  which  effects  its  consumption. 

The  Liver. 

The  liver  is  the  largest  gland  in  the  body.  Its  function  is  to 
produce  bile,  glycogen  and  urea. 

Anatomy, — The  liver  is  situated  in  the  upper  part  of  the 
abdominal  cavity,  chiefly  in  the  right  hypochondrium.  Its 


FIG.  43. — The  under  surface  of  the  liver. 

g.  b.,  gall  bladder;  h.  d.,  common  bile  duct-  h.  a.,  hepatic  artery;  v.  p.,  portal  vein; 
/.  q.,  lobulus  quadratus;  /.  s.,  lobulus  spigelii;  /.  c.,  lobulus  caudatus;  d.  v.,  ductus 
venosus;  u.  v.,  umbilical  vein.  (Kirkes  after  Noble  Smith.) 

weight  in  the  average  adult  is  about  four  and  a  half  pounds.  It 
is  covered,  except  for  a  small  area  behind,  by  peritoneum, 
processes  of  which  run  from  it  at  several  points  and  constitute 
its  supporting  ligaments.  The  proper  coat  of  the  liver  lies 


THE    LIVER  105 

underneath  the  peritoneum,  and  at  the  transverse  fissure  is 
continued  into  the  gland  as  a  sheath,  embracing  the  structures 
entering  there  and  ramifying  with  them  in  their  distribution. 
This  is  the  capsule  of  Glisson.  It  is  fibrous  in  structure,  is 
closely  attached  to  the  liver  substance,  and  rather  loosely  ad- 
herent to  the  structures  which  it  envelops.  The  walls  of  the 
portal  vein  are  seen  collapsed  on  section,  while  those  of  the 
hepatic  veins,  which  are  not  surrounded  by  Glisson's  capsule, 
and  which  are  closely  adherent  to  the  gland  substance,  stand 
well  open. 

A  general  idea  of  the  liver's  anatomy  is  obtained  by  noting 
that  it  has  five  lobes,  five  fissures,  five  ligaments  and  five  struc- 
tures passing  through  the  transverse  fissure.  The  lobes  are  right, 
left,  caudate,  quadrate  and  Spigelian.  The  fissures  are  trans- 
verse, umbilical,  that  for  the  ductus  venosus,  the  fossa  for  the 
vena  cava  and  the  fossa  vesicalis.  The  ligaments  are  coronary, 
right  lateral,  left  lateral,  round  and  suspensory  or  longitudinal. 
The  structures  passing  through  the  transverse  fissure  are  the 
portal  vein,  the  hepatic  artery,  the  hepatic  duct,  the  nerves  and 
the  lymphatics. 

Blood-vessels. — Of  the  two  blood-vessels  entering  the  fissure 
the  portal  vein  is  decidedly  the  larger.  It  has  collected  the 
blood  from  the  abdominal  organs  by  the  radicles  of  its  tribu- 
taries, the  gastric,  splenic,  superior  and  inferior  mesenteric  veins, 
while  the  hepatic  artery  is  a  branch  of  the  celiac  axis.  These, 
having  been  distributed  in  a  manner  to  be  noted  presently,  dis- 
charge their  blood  into  the  radicles  of  the  hepatic  veins,  which, 
usually  three  in  number,  enter  the  ascending  vena  cava,  where 
that  vessel  passes  through  the  liver  behind.  Again,  it  is  to  be 
remembered  that  these  two  vessels,  as  well  as  the  nerves  and 
lymphatics,  are  enveloped  in  the  vagina,  or  capsule  of  Glisson. 

The  portal  vein  and  the  hepatic  artery  give  off  branches  to 
the  capsule  of  Glisson,  constituting  the  vaginal  plexus.  The 
portal  vein,  still  ensheathed,  then  divides  and  subdivides  until 


io6 


THE    PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 


its  branches  run  directly  between  the  lobules,  and  are  called  in- 
terlobular  veins.  These  direct  subdivisions  of  the  portal  vein 
are  not  the  only  interlobular  veins,  however.  Those  branches 
of  this  vein  which  were  given  off  to  the  capsule  of  Glisson, 
having  received  the  corresponding  branches  from  the  hepatic 
artery,  also  here  run  between  the  lobules  and  make  part  of  the 


FIG.  44. — Diagram  of  the  portal  vein. 

(pv)  arising  in  the  alimentary  tract  and  spleen  (s)  and  carrying  the  blood  from 
these  organs  to  the  liver.     (From  Brubaker  after  Yeo.) 

interlobular  plexus.  The  interlobular  veins,  thus  surrounding 
the  lobules  and  having  lobules  on  either  side  of  them,  giving  off 
in  both  directions  branches  (lobular  branches)  which  penetrate 
the  lobules,  to  break  up  into  capillaries.  The  capillaries  finally 
converge  to  three  or  four  small  radicles,  which  in  turn  unite  to 
form  a  small  vein  in  the  center  of  the  lobule.  This  is  the  in- 


THE    LIVER 


107 


tralobular  vein,  which  at  the  base  of  the  lobule  joins  the  sub- 
lobular  vein.     These  sublobular  veins  join  each  other  to  form 


FIG.  45. — Section  of  lobule  of  liver  of  rabbit  in  which  the  blood  capillaries 
and  bile  canaliculi  have  been  injected.     (From  Yeo  after  Cadiat.) 

a,  intralobular  vein ;  b,  interlobular  veins;  c,  biliary  canals  beginning  in  fine  capillaries. 

hepatic  veins,  which  become  larger  and  larger  until  they  have 
collected  all  the  blood  which  has  entered  the  liver.  They  finally 
enter  the  ascending  vena  cava. 


108         THE   PHYSIOLOGY   OF   DIGESTION  AND  ABSORPTION 

But  what  has  become  of  the  hepatic  artery?  As  soon  as  it  has 
entered  the  sheath,  it  gives  off  branches  to  the  capsule  forming 
part  of  the  vaginal  plexus  and  entering  into  the  vaginal  branches 
of  the  portal  vein  just  before  these  run  between  the  lobules.  It 
also  furnishes  branches  to  the  wall  of  the  portal  vein,  to  the  wall 
of  the  larger  divisions  of  the  artery  itself,  and  to  the  hepatic  duct. 

Histology  of  a  Lobule. — The  liver  is  made  up  of  a  large 
number  of  lobules  about  one-twenty-fifth  of  an  inch  in  diameter, 
separated  by  vessels,  nerves  and  radicles  of  the  hepatic  duct. 
Such  a  lobule  in  certain  of  the  lower  animals  has  a  distinct  poly- 
gonal shape,  but  in  man  the  outlines  are  not  clear.  In  the  lobule 
are  the  hepatic  cells,  ovoid  in  shape,  possessed  of  small  granules 
and  one  or  two  nuclei.  They  are  disposed  in  columns  radiating 
from  the  central  intralobular  vein.  These  cells  belong  to  the 
epithelial  type,  and  the  liver  is  not  essentially  different  from 
other  glands,  such  as  the  salivary,  except  in  the  complexity  of  its 
arrangement.  The  analogy  is  established  by  the  origin  of  the 
bile  ducts  in  the  lobules  between  the  cells. 

Bile  Ducts.— It  is  not  difficult  to  demonstrate  the  interlobu- 
lar  ducts,  but  to  follow  them  as  such  into  the  lobule  is  less  easy. 
However,  there  is  no  doubt  at  all  that  they  do  originate  between 
the  hepatic  cells.  It  is  probable  that  here  they  have  no  distinct 
lining  membrane,  but  are  mere  tubular  intercellular  spaces, 
into  which  the  bile  is  poured  and  carried  into  the  interlobular 
duct.  Typically  a  liver  cell  has  one  of  these  bile  capillaries  on 
one  side  and  a  blood  capillary  on  the  other,  and  while  this  rela- 
tion does  not  always  hold  good,  every  cell  does  communicate 
with  both  kinds  of  capillaries.  The  interlobular  bile  ducts  con- 
sist of  epithelium  resting  upon  a  very  thin  basement  membrane. 
As  they  increase  in  size  they  gain  fibrous  inelastic  and  elastic 
tissue,  and  the  largest,  some  non-striated  muscular  elements. 
Gradually  as  the  ducts  become  larger  the  lining  epithelium 
changes  from  the  columnar  to  the  pavement  form.  Mucous 
glands  exist  in  the  largest  ducts.  The  interlobular  ducts  join 


THE    LIVER 


ICQ 


each  other  and  gradually  increase  in  size  as  they  merge  from  all 
parts  of  the  liver,  to  leave  its  substance  in  two  divisions — one 
from  the  right  and  one  from  the  left  lobe.  These  two  unite  to 
form  the  hepatic  duct  which,  running  a  course  of  about  one  and 
a  half  inches,  is  joined  at  an  acute  angle  by  the  cystic  duct  to 
form  the  common  bile  duct,  or  the  ductus  communis  chole- 
dochus.  This  last  penetrates  obliquely  the  duodenal  wall  and 


Branch  of  portal  vein. 


Large  interlocular 
bile  duct. 


Interlobular  con- 
nective tissue. 


FIG.  46. — From  a  horizontal  section  of  human  liver.     X4O. 

Three  central  veins,  cut  transversely,  represent  each  a  center  of  as  many  hepatic 
lobules,  that  at  the  periphery  are  but  slightly  defined  from  their  neighbors.  Below 
and  to  the  right  of  the  section  the  lobules  are  cut  obliquely  and  their  boundaries 
cannot  be  distinguished.  (From  Stohr.) 

discharges  the  bile  into  the  intestine.  The  cystic  duct  has  its 
origin  at  the  apex  of  the  gall  bladder,  and  is  about  one  inch  long. 
The  common  bile  duct  has  an  average  length  of  three  inches. 
(See  Fig.  43.) 

Gall  Bladder.— The  gall  bladder  has  an  oval  shape  with  its 
large  end  forward.  It  is  on  the  under  surface  of  the  liver,  the 
^peritoneum  running  over  (or  rather  under)  it.  It  has  a  mucous 


110         THE    PHYSIOLOGY    OF   DIGESTION  AND  ABSORPTION 

lining  and  the  remainder  of  its  structure  is  chiefly  fibrous.  A 
little  plain  muscular  tissue  may  exist.  Its  capacity  is  about  one 
and  a  half  ounces.  Mucous  glands  are  found  in  its  lining,  as  in 
that  of  the  large  ducts,  and  these  are  responsible  for  the  mucin 
of  the  bile. 

Hepatic  Nerves. — With  regard  to  the  exact  destination  of 
the  nerves  entering  the  liver,  little  is  known.  Evidence  going 
to  establish  the  termination  of  fibers  in  the  cells,  that  is,  the 
existence  of  distinct  secretory  fibers,  is  meager.  There  is  little 
doubt  that  secretory  fibers  for  the  glycogenic  function  of  the 
liver  do  exist.  It  is  known  that  fibers  from  the  vagus,  phrenic 
and  solar  plexus  enter  the  fissure,  but  they  cannot  be  followed 
farther  than  the  ramifications  of  Glisson's  capsule  between  the 
lobules.  Of  course,  vaso-motor  fibers  go  to  the  vessels,  as  else- 
where. Fibers  acting  similarly  go  also  to  the  muscular  tissue  of 
the  large  ducts  and  of  the  gall  bladder.  The  contraction  of  the 
gall  bladder  is  thought  to  be  reflex,  afferent  impressions  being 
conveyed  by  the  vagus  from  the  mucous  membrane  of  the 
duodenum. 

Hepatic  Lymphatics. — The  lymphatics  are  abundant,  and 
those  not  originating  in  the  connective  tissue  are  thought  to 
originate  by  perivascular  canals  surrounding  the  blood-vessels 
of  the  lobules.  The  fact  that  when  the  outflow  of  bile  is  occluded 
it  passes,  not  into  the  vascular,  but  into  the  lymphatic  circulation 
is  a  curious  circumstance.  It  may  be  due  to  the  absence  of  a 
definite  wall  for  the  intralobular  ducts  and  their  comparatively 
free  communication  with  the  lymphatics  in  those  localities. 

Properties  and  Composition  of  Bile. — Human  bile  is  of  a 
dark  greenish-red  color,  has  a  bitter  taste  and  is  practically  odor- 
less when  fresh.  It  undergoes  putrefaction  easily,  but  is  not 
coagulable  by  heat.  It  is  viscid,  chiefly  on  account  of  the  mucin 
it  contains.  It  has  an  alkaline  reaction,  and  a  specific  gravity 
of  about  1030.  Besides  water,  which  constitutes  more  than 
ninety  per  cent,  of  its  bulk,  it  contains  the  sodium  salts  of  tauro- 


THE    LIVER  III 

cholic  acid  and  glycocholic  acid  (the  biliary  salts), cholesterin, 
bilirubin,  lecithin,  fats,  soaps,  mucin  and  various  inorganic 
salts,  such  as  sulphates,  carbonates,  phosphates,  etc.,  and  a 
quantity  of  carbon  dioxide.  The  quantity  of  bile  secreted  in 
twenty-four  hours  is  about  two  and  a  half  pounds. 

In  human  bile  sodium  taurocholate  largely  predominates 
over  glycocholate.  These  are  formed  as  acids  by  the  liver  cells, 
are  absorbed  in  their  passage  down  the  intestine,  and  are  pre- 
sumably those  parts  of  the  bile  which  are  concerned  in  its  digestive 
action,  particularly  in  the  absorption  of  fats.  So  far  as  these 
constituents  are  concerned,  the  bile  is  a  typical  secretion. 

Cholesterin,  on  the  other  hand,  seems  to  be  simply  removed 
from  the  blood  by  the  liver  cells,  and  is  discharged  in  the  feces, 
where,  however,  it  exists  in  a  slightly  changed  form,  stercorin. 
It  is  thought  to  be  held  in  solution  by  the  bile  acids,  glycocholic 
and  taurocholic.  So  far  as  this  constituent  is  concerned,  there- 
fore, the  bile  is  a  typical  excretion.  It  is  produced  in  many  of 
the  body  tissues,  and  no  function  has  been  discovered  for  it. 

Bilirubin  is  the  characteristic  coloring  matter  of  the  human 
bile ;  that  of  herbivorous  animals  is  biliverdin,  and  a  little  of  this 
latter  is  also  present  in  human  bile.  These  pigments  originate 
from  hemoglobin.  It  is  supposed  that  when  the  red  corpuscles 
break  down,  "the  hemoglobin  is  brought  to  the  liver,  and  then 
under  the  influence  of  the  liver  cells  is  converted  into  an  iron-free 
compound,  bilirubin,  or  biliverdin."  (Howell.) 

The  lecithin  is  probably  an  end  product  of  physiological 
activity  in  the  tissues,  and  is  apparently  an  excretion. 

The  mucin  gives  the  fluid  its  viscid  character. 

The  production  of  bile  is  continuous,  but  this  does  not  mean 
that  its  discharge  into  the  duodenum  is  continuous,  for  in  the 
intervals  of  digestion  it  is  not  admitted  (freely  at  least)  into  the 
intestine,  but  regurgitates  from  the  ductus  communis  choledo- 
chus  through  the  cystic  duct  into  the  gall  bladder,  which  acts  as  a 
reservoir  until  its  contents  are  needed.  The  secretion  is  more 


112         THE    PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

active,  however,  during  intestinal  digestion  than  at  other  times. 
This  appears  to  be  reflex,  but  may  be  simply  a  result  of  the  in- 
creased amount  of  blood  passing  through  the  portal  vein  to  the 
liver  during  that  period,  for  the  whole  alimentary  canal  is  con- 
gested while  digestive  activity  is  in  progress.  Again,  it  is  known 
that  the  best  cholagogue  is  bile  itself,  and  some  of  the  bile  is 
absorbed  in  its  passage  down  the  intestine.  Its  presence  in  the 
blood  may  account  for  the  accelerated  flow. 

Method  of  Secretion  and  Discharge.— The  bile  is  a  product 
of  the  liver  cells.  How  they  receive  their  normal  stimulus  is 
obscure.  But  it  is  reasonable  to  suppose  that  a  larger  supply  of 
blood  means  a  more  abundant  secretion.  Such  an  increase  of 
blood  supply  occurs  during  digestion. 

The  cells  discharge  the  bile  into  the  bile  capillaries,  which 
pass  it  onward  either  to  the  intestine  directly,  or,  during  the 
intervals  of  digestion,  to  the  gall  bladder.  When  food  enters 
the  duodenum,  a  reflex  influence  causes  the  wall  of  the  gall 
bladder  to  contract  and  compress  its  contents.  The  only  out- 
let is  through  the  cystic  duct  into  the  common  duct,  and  thence 
into  the  duodenum.  This  reflex  does  not  take  place  until 
food  has  entered  the  duodenum,  and  of  different  foods  it  is 
found  that  proteids  (peptones)  and  fats  are  the  most  efficient 
stimuli. 

The  secretion  of  bile  is  not  stopped  by  ligation  of  either  the 
portal  vein  or  the  hepatic  artery,  showing  that  both  of  these 
vessels  contain  bile  materials.  But  it  would  be  unreasonable  to 
suppose  that  the  blood  of  the  portal  vein  does  not  furnish  the 
bulk  of  secreting  material. 

Glycogenic  Function. — The  formation  of  glycogen  is  con- 
nected with  nutrition,  but  will  receive  some  notice  here.  This 
is  an  internal  secretion.  It  is  produced  by  the  liver  cells,  and 
can  be  demonstrated  in  their  substance  by  the  microscope  and 
by  chemical  reagents.  It  can  also  be  shown  to  increase  markedly 


THE    LIVER  113 

after  eating,  and  to  decrease  notably  when  eating  is  refrained 
from  for  some  time. 

Glycogen  is  a  carbohydrate  very  similar  to  starch,  and  when 
ingested  it  is  acted  upon  by  the  same  enzymes  and  undergoes 
the  same  conversions.  Furthermore,  the  amount  of  glycogen 
in  the  liver  is  very  greatly  increased  by  restricting  the  diet  to 
carbohydrate  foods  and  is  lessened  considerably  below  the  nor- 
mal (that  is,  its  amount  on  a  mixed  diet),  but  is  not  reduced  to 
zero,  when  proteids  alone  are  taken.  This  points  to  the  con- 
clusion that  the  source  of  glycogen  is  carbohydrates,  but  that  it 
can  be  formed  to  some  extent  from  proteids.  Let  it  be  said 
now  that  practically  all  carbohydrates  are  converted  by  digestion 
into  maltose,  or  maltose  and  dextrin  and  furthermore  that  during 
absorption  these  sugars  are  converted  into  dextrose  or  dextrose 
and  levulose.  It  is  customary  to  assume  that  the  digestion  of  a 
carbohydrate  means  its  conversion  into  dextrose  (glucose,  levu- 
lose). It  is,  then,  this  sugar  which  is  carried  to  the  liver  by  the 
portal  vein. 

We  may  say  that  the  formula  for  dextrose  is  C6H]2O6  and  for 
glycogen  C6H10O5,  though  neither  of  these  formulae  is  probably 
exactly  correct.  It  will  be  seen,  therefore,  that  the  abstraction  of 
one  molecule  of  water  (H2O)  from  dextrose  will  produce  glyco- 
gen, and  this  is  the  change  which  the  liver  cells  are  supposed  to 
effect.  Again,  when  the  conversion  of  dextrose  into  glycogen 
has  taken  place,  the  glycogen  is  stored  up  in  the  liver  cells,  to  be 
given  off  continuously  to  the  blood  only  in  such  quantities  as  the 
system  may  demand.  The  liver  thus  becomes  a  warehouse  for 
the  storage  of  all  the  carbohydrates. 

It  will  be  seen  under  Nutrition  that  the  carbohydrates  furnish 
the  chief  material  to  be  burned  up  in  the  body  for  the  purpose 
of  liberating  heat  and  furnishing  energy,  and  if  they  should  be 
consumed  as  soon  as  they  enter  the  circulation,  there  would  be 
not  only  an  unnecessary  waste  during  Iheir  quick  consumption, 
but  also  an  unfortunate  lack  of  energy-producing  materials  before 


114         THE    PHYSIOLOGY   OF    DIGESTION  AND   ABSORPTION 

another  meal.  This  storing  up  brings  about  a  kind  of  conserva- 
tion of  energy  and  an  economical  regulation  of  its  distribution. 
The  amount  of  sugar  in  the  circulation  at  any  time  is  very  small, 
and  a  single  carbohydrate  meal  may,  by  the  action  of  the  liver, 
be  made  to  supply  the  carbohydrate  demands  of  the  tissues  for 
a  considerable  period. 

Now,  it  was  just  said  that  the  sugar  of  the  blood  is  dextrose;  if 
the  dextrose  of  the  portal  blood  is  converted  into  glycogen  to  be 
stored  up,  it  must  be  reconverted  into  dextrose  before  it  can  leave 
the  liver,  since  it  leaves  by  the  blood.  The  cells  do  effect  the 
second  conversion,  and  this  is  the  second  part  of  the  glycogenic 
function.  It  may  be  that  the  liver  cells  produce  an  enzyme 
corresponding  to  ptyalin,  which  converts  the  glycogen.  Dex- 
trose does  not  normally  exist  in  the  liver  cells.  At  the  very 
moment  of  its  formation  it  is  carried  away  by  the  blood. 

The  fact  that  the  liver  can  form  glycogen  out  of  proteids  shows, 
of  course,  that  the  nitrogen  is  eliminated  from  the  proteid  mole- 
cule in  some  way.  A  carbohydrate  molecule  is  left  to  be  oxidized 
in  the  usual  manner.  This  is  thought  to  be  the  initial  step  in 
the  final  consumption  of  proteids  in  nutrition.  The  fats  have  no 
influence  on  glycogen  formation. 

Glycogen  also  exists  in  other  parts  of  the  body,  particularly 
in  the  voluntary  muscular  substance.  The  cells  of  the  tissue  in 
which  it  is  found  must  also  have  a  glycogenic  function. 

Urea  Formation. — But  the  liver  has  another  function  besides 
the  production  of  bile  and  glycogen,  and  that  is  to  form  urea. 
It  will  be  seen  later  that  the  chief  end  product  of  proteid  metab- 
olism is  urea,  and  that  it  is  eliminated  almost  entirely  by  the 
kidneys.  The  liver  is  much  more  active  in  the  production  of 
this  substance  when  the  portal  blood  is  charged  with  digested 
materials,  but  it  also  forms  urea  in  fasting  animals.  The  liver 
must,  therefore,  be  capable  of  forming  urea  from  some  of  the 
products  of  digested  foods.  With  reference  to  its  formation  in 
fasting  animals,  suffice  it  to  say  here  that  it  seems  that  as  long  as 


THE    INFLUENCE    OF    THE    BILE    ON    DIGESTION  115 

proteid  metabolism  goes  on  in  other  tissues,  there  are  produced, 
in  those  tissues  materials  (ammonia  compounds)  which,  when 
carried  to  the  liver,  are  converted  by  it  into  urea.  Further  notice 
will  be  given  to  this  phase  of  the  subject  under  Nutrition. 

The  liver  cells  produce  urea;  it  enters  the  blood,  is  carried  to 
the  kidneys  and  eliminated  by  those  organs.  In  the  mechanism 
of  its  production  and  discharge  from  the  liver,  it  thus  corresponds 
to  the  internal  secretions,  though  urea  is  distinctly  an  excretion. 

It  must  not  be  supposed,  however,  that  the  liver  is  the  only 
organ  producing  urea.  There  are  other  organs  which  certainly 
produce  it,  while  there  are  those  who  maintain  that  it  is  produced 
directly  wherever  proteid  metabolism  is  in  progress. 

The  Influence  of  the  Bile  on  Digestion 

The  bile  is  not,  properly  speaking,  a  digestive  fluid,  for  it 
contains  no  enzyme  capable  of  effecting  digestive  changes  in 
any  of  the  foods;  but  it  so  materially  affects  the  action  of  some 
of  the  other  fluids  that  it  cannot  be  overlooked  in  a  discussion 
of  intestinal  digestion. 

So  far  as  the  bile  acids,  glycocholic  and  taurocholic  (com- 
bined to  form  salts  of  sodium)  are  concerned,  the  fluid  is  a  se- 
cretion, and  it  is  these  which  are  mainly  concerned  in  the  diges- 
tive process.  The  production  of  bile  is  continuous,  but  the 
gall  bladder  acts  as  a  reservoir  in  which  a  part  at  least  of  the 
secretion  is  stored  in  the  intervals  of  digestion,  to  be  discharged 
in  greater  abundance  when  chyme  enters  the  duodenum.  While 
the  action  of  bile  in  most  of  the  digestive  functions  to  be  men- 
tioned is  obscure,  it  is  known  to  have  at  least  these  uses: 

1.  It  promotes  intestinal  peristalsis. 

2.  It  has  an  inhibitory  effect  on  putrefaction  in  the  intestinal 
tract.     By  this  it  is  not  to  be  understood  that  the  bile  is  directly 
antiseptic,  for  it  undergoes  putrefaction  very  readily  itself,  but 
only  that  in  some  way  its  withdrawal  from  the  substances  passing 


Il6         THE   PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

through  the  alimentary  canal  allows  their  more  ready  disinte- 
gration. 

3.  It  aids  in  the  emulsification  of  fats. 

4.  It  promotes  the  absorption  of  fats.     Recently  the  state- 
ment that  the  bile  promotes  all  kinds  of  absorption  has  appar- 
ently been  successfully  disproved,  but  it  seems  certain  that  "the 
bile  acids  enable  the  bile  to  hold  in  solution  a  considerable 
quantity  of  fatty  acids,  and  possibly  this  fact  explains  its  connec- 
tion with  fat  absorption."     (American  Text-Book.) 

The  Secretion  of  the  Intestines. 

The  intestinal  secretion,  or  succus  entericus,  is  a  product  of 
the  crypts  of  Lieberkuhn  and  Brunner's  glands.  It  is  scanty,  of 
a  yellow  color  and  an  alkaline  reaction.  Opinions  vary  as  to 
what  foods  are  affected  by  this  fluid,  but  since  the  more  recent 
experiments  have  overcome  some  difficulties  in  obtaining  speci- 
mens, the  conclusions  based  upon  them  seem  most  reliable.  It  is 
said  to  have  no  effect  on  proteids  or  fats.  It  contains  an  amylo- 
lytic  enzyme,  which  aids  the  pancreatic  juice  in  converting 
starch  into  maltose.  It  also  has  an  enzyme,  invertase,  which 
converts  cane  sugar  into  dextrose  and  levulose,  as  well  as  an  allied 
enzyme,  maltase,  which  converts  maltose  into  dextrose.  The 
carbohydrates  are  absorbed  as  dextrose,  with  the  probable  excep- 
tion of  lactose.  It  is  mainly  cane  sugar,  maltose  (from  starch) 
and  lactose  that  are  in  the  alimentary  tract  and  require  to  be  thus 
changed  to  dextrose. 

It  is  not  out  of  place  to  say  that  ptyalin  produces  maltose  and  a 
little  dextrose,  and  that  the  pancreatic  juice  and  succus  entericus 
produce  maltose  and  considerable  dextrose.  The  maltose  is 
converted  into  dextrose  during  the  process  of  absorption.  It  is, 
therefore,  customary  to  say  that  the  carbohydrates  are  absorbed 
only  as  dextrose. 

Movements  of  the  Small  Intestine.— The  effect  of  intestinal 
movements  is  to  force  the  contents  onward  through  the  ileocecal 


LARGE   INTESTINE  II  7 

valve.  Here  it  is  that  typical  peristalsis  is  found.  The  main 
factor  in  the  passage  is  the  layer  of  circular  fibers.  Contraction 
of  these  fibers  in  the  upper  duodenum  may  at  least  be  conceived 
to  begin  upon  the  introduction  of  chyme.  The  contraction 
passes  down  the  gut  in  a  wave-like  manner,  the  wave  being 
produced  by  the  contraction  of  segment  after  segment  of  the 
circular  fibers  with  relaxation  just  behind  the  advancing  con- 
traction. The  tendency  of  such  a  movement  is  to  force  the 
alimentary  mass  along  the  canal.  The  longitudinal  fibers  are 
probably  chiefly  concerned  in  changing  the  position  of  the  intes- 
tine and  in  shortening  the  tube,  and  thus  slipping  the  mucous 
membrane  above  the  bolus,  so  that  it  can  be  grasped  by  the 
circular  fibers.  A  continuation  and  repetition  of  these  move- 
ments, which  are  slow,  gentle  and  gradual  in  character,  is  finally 
effectual  in  passing  the  contents  into  the  colon.  It  is  not 
probable  that  antiperistaltic  movements  take  place  normally. 

Nerve  Supply. — Very  probably  the  intestinal  movements  are 
naturally  excited  by  the  food  and  by  the  bile.  It  is  probable  also 
that  these  stimuli  exert  their  influence  through  the  ganglia  of  the 
plexuses  of  Auerbach  and  Meissner.  The  intestine  receives 
fibers  from  the  right  vagus  and  the  sympathetic.  The  former 
are  probably  motor  (contractors)  and  the  latter  inhibitory 
(dilators).  Here,  as  in  the  stomach,  they  are  probably  only 
regulators  of  the  movements,  without  being  actually  necessary  to 
peristalsis. 

The  Large  Intestine. 

Anatomy. — The  large  intestine,  known  as  the  colon,  is 
about  five  feet  in  length  and  is  divided  into  ascending,  trans- 
verse and  descending  portions.  The  sigmoid  flexure  is  the 
terminal  extremity  of  the  descending  colon  and  empties  into 
the  rectum.  The  small  intestine  communicates  with  the  colon 
at  right  angles  a  little  above  the  beginning  of  the  latter,  leaving 
below  the  opening  a  blind  pouch,  the  cecum,  or  caput  coli. 


Il8         THE   PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

From  the  posterior  and  inner  aspect  of  the  cecum  runs  off  the 
appendix  vermiformis.  The  diameter  of  the  colon  gradually 
decreases  from  two  and  a  half  to  three  and  a  half  inches  in  the 
cecum  to  the  beginning  of  the  rectum.  The  ascending  colon 
passes  upward  from  its  beginning  in  the  right  iliac  fossa  to  the 
under  surface  of  the  liver,  where  it  bends  upon  itself  almost  at  a 
right  angle  (hepatic  flexure).  The  transverse  colon  runs  di- 
rectly across  the  upper  part  of  the  abdominal  cavity  to  the  lower 
border  of  the  spleen,  where  an  abrupt  turn  downward  (splenic 
flexure)  begins  the  descending  colon.  The  lower  part  of  the 
descending  colon  occupies  the  left  iliac  fossa  in  the  shape  of  the 
letter  S,  and  is  the  sigmoid  flexure. 

The  rectum,  which  receives  the  contents  of  the  sigmoid,  is 
not  straight,  as  its  name  indicates.  It  curves  (i)  to  the  right  to 
reach  the  median  line,  (2)  forward  to  follow  the  contour  of 
the  sacrum,  and  (3)  backward  in  the  last  inch  of  its  course. 
It  has  the  shape  of  a  dilated  pouch,  its  lower  termination  at  the 
anus  being  guarded  by  the  powerful  external  sphincter  of  stri- 
ated muscle.  Its  diameter  is  greatest  below. 

The  vermiform  appendix  has  the  three  coats  common  to  the 
intestine,  but  its  muscular  coat  is  ill-developed.  The  peritoneal 
coat  generally  forms  a  short  meso-appendix  at  the  root  of  the 
organ.  The  blood  supply  of  the  organ  is  not  abundant.  It  is 
greater  in  the  female  than  in  the  male,  a  part  of  it  coming 
through  the  appendiculo-ovarian  ligament.  The  appendix  has 
no  function. 

The  ileo-cecal  valve,  guarding  the  opening  between  the  large 
and  small  intestines,  is  made  of  two  folds,  upper  and  lower,  of  the 
muscular  and  mucous  coats,  which  folds  project  into  the  large 
intestine.  The  serous  coat  runs  directly  over  from  the  small 
to  the  large  intestine  at  their  point  of  junction,  without  being 
folded  inward  upon  itself,  as  are  the  others.  This  prevents 
obliteration  of  the  folds  by  distention.  By  this  arrangement  the 
two  portions  of  the  gut  communicate  only  by  a  buttonhole  slit, 


LARGE    INTESTINE  Iig 

which  is  easily  opened  by  pressure  from  the  direction  of  the 
ileum  but  which  pressure  from  the  cecum  tends  to  close  more 
firmly. 

Structure. — The  large  intestine  has  the  usual  three  coats. 
The  peritoneal,  however,  is  lacking  on  the  posterior  part  of  the 
cecum,  ascending  and  descending  colons,  these  parts  being 
bound  down  closely  and  having  no  meso-colon.  The  sigmoid 
is  entirely  covered  as  is  the  upper  third  of  the  rectum.  The 
middle  third  of  the  rectum  has  no  serous  coat  behind,  being 
firmly  held  in  place,  while  the  lower  third  lacks  this  coat  en- 
tirely. The  muscular  coat  is  peculiar,  in  that  its  longitudinal 
fibers  are  collected  into  three  quite  strong  bands,  evident  to  the 
eye.  When  the  rectum  is  reached  they  spread  out  over  the 
whole  circumference  of  that  part  of  the  canal.  These  bands  are 
shorter,  as  it  were,  than  the  wall  proper,  and  the  consequence  is 
that  the  whole  length  of  the  large  intestine  is  gathered  up  into 
a  number  of  pouches.  The  mucous  coat  is  paler  than  that  of 
the  small  intestine,  presents  no  villi  and  is  rather  closely  adher- 
ent to  the  subjacent  parts.  In  it  are  found  glands  corresponding 
in  appearance  to  the  crypts  of  Lieberkuhn,  and  they  are  so 
classed;  but  they  probably  secrete  mucus  only.  Some  solitary 
lymphoid  follicles  also  usually  exist  here. 

Changes  Taking  Place  in  the  Alimentary  Mass  in  the 
Large  Intestine. — Most  of  the  substances  which  enter  the  large 
intestine  have  resisted  the  action  of  the  various  digestive  fluids 
and  are  on  their  way  to  be  discharged  in  defecation.  Doubtless, 
though,  some  materials  undergo  digestive  changes  in  the  colon, 
not  under  the  influence  of  any  secretion  there  formed,  but  of 
the  intestinal  juice  with  which  they  are  incorporated  on  leaving 
the  ileum.  The  secretion  of  the  mucous  membrane  of  the  large 
intestine  furnishes  no  digestive  enzyme,  and  the  changes  going 
on  in  the  alimentary  mass  (now  feces)  are  chiefly  due  to  absorp- 
tion. By  some  unknown  process,  however,  rectal  aliments 
of  an  easily  digestible  nature  are  absorbed,  and  that  in  a  nutritive 


120         THE   PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

form.  The  consistence  of  the  fecal  matter  increases  in  its  pas- 
sage through  the  colon,  owing  to  the  absorption  of  its  more  fluid 
portions.  The  bile  pigment  is  responsible  for  the  character- 
istic color.  The  odor  is  mainly  due  to  bacterial  decomposition, 
but  partly  to  the  secretion  of  the  mucous  membrane. 

Bacteria  in  Intestinal  Digestion. — The  entrance  of  the  bile 
and  pancreatic  juice  into  the  duodenum  changes  to  alkaline 
the  previously  acid  reaction  of  the  chyme.  But  it  is  found  that, 
when  an  ordinary  mixed  diet  is  given,  the  mass  leaving  the  ileo- 
cecal  valve  has  an  acid  (proteid)  reaction,  and  that  the  proteids 
have  not  undergone  putrefaction.  The  alkaline  medium  of  the 
upper  intestine  favors  bacterial  activity,  and  it  would  seem  that 
proteid  putrefaction  would  ensue.  But  it  is  supposed  that  in 
health  these  bacteria  set  up  fermentative  changes  in  the  carbohy- 
drates, with  the  production  of  acids  which  inhibit  proteid  putre- 
faction, and  account  for  the  acid  reaction  at  the  ileo-cecal  valve. 
When  the  mass  has  entered  the  colon  the  acidity  is  soon  overcome 
and  putrefaction  is  the  Usual  consequence.  It  can  be  seen  how 
readily  this  delicately  adjusted  balance  may  be  disturbed  by 
errors  in  the  proper  kind  and  proportion  of  food,  etc.  Some  of 
the  products  of  bacterial  activity  upon  carbohydrates  and  proteids 
are  leucin,  tyrosin,  indol,  skatol,  phenol,  lactic  and  butyric  acid. 
The  object  of  the  production  of  these  substances  is  unknown. 

Composition  of  Feces. — It  seems  at  present  that  the  main 
bulk  of  fecal  matter  is  made  up  of  substances  which  are  contained 
in  the  intestinal  secretions,  and  the  alimentary  canal  is  more  im- 
portant in  excretion  than  was  formerly  supposed.  These  sub- 
stances are  waste  matters  from  tissue  metabolism.  Besides  these 
materials,  the  feces  normally  contain  indigestible  and  undigested 
matters,  inactive  salts,  stercorin,  mucus,  epithelium  from  the 
intestinal  wall,  coloring  matter  and  substances  resulting  from 
bacterial  activity.  Stercorin  is  the  converted  form  of  cholesterin, 
a  constituent  of  the  bile.  The  coloring  matter  is  from  the 
pigment  (bilimbiri)  of  the  same  fluid.  Of  the  bacterial  products 


LARGE    INTESTINE  121 

the  most  important  are  indol  and  skatol.  They  represent  pro- 
teid  putrefaction;  they  are  responsible  for  the  fecal  odor;  hence 
the  characteristic  difference  in  the  odor  of  the  contents  of  the 
ileum  and  colon.  The  reaction  of  fecal  matter  varies.  The 
amount  for  the  average  person  is  about  four  and  a  half  ounces 
per  day. 

Gases. — Hydrogen,  nitrogen  and  carbon  dioxide  are  found 
normally  in  the  small  intestines.  They  serve  to  keep  the  tube 
patulous,  and  avoid  obstruction,  and  also  to  prevent  concussion. 
In  the  large  intestine  bacterial  activity  increases  the  number  of 
gases  present.  Here,  in  addition  to  those  found  in  the  small 
intestine,  there  are  carbureted  and  sulphuretted  hydrogen,  with 
others  at  times. 

Movements  of  the  Large  Intestine. — The  muscular  contrac- 
tions of  the  colon  forcing  the  feces  onward  are  of  the  same  general 
character  as  those  of  the  small  intestine,  though  less  violent. 
The  contents  thus  passed  analward  by  peristalsis  accumulate 
gradually  in  the  sigmoid  flexure  until  defecation  occurs. 

Defecation. — The  act  of  defecation  is  both  voluntary  and  in- 
voluntary— voluntary  in  the  relaxation  of  the  external  sphincter 
and  involuntary  in  the  peristalsis  which  brings  the  fecal  matter 
to  present  at  that  muscle.  It  is  probable  that  the  rectal  pouch 
does  not  usually  contain  feces,  but  that  the  desire  to  defecate  is 
brought  about  by  the  entrance  of  the  mass  into  it  from  the  sig- 
moid. Then,  if  the  desire  is  obeyed,  peristalsis  of  the  non- 
striated  muscular  coat  continues,  the  internal  sphincter  of  plain 
muscle  relaxes,  as  does  also  the  external  of  striped  muscle,  and 
evacuation  takes  place. 

Usually,  by  an  effort  of  the  will,  evacuation  can  be  voluntarily 
prevented  by  maintaining  the  tonic  contraction  of  the  external 
sphincter.  If  the  desire  to  defecate  be  disregarded,  the  fecal 
accumulation  probably  returns  to  the  sigmoid,  leaving  the  rec- 
tum comparatively  empty.  The  act  of  evacuation  is  commonly 
aided  further  by  voluntary  contraction  of  the  diaphragm  and 


122          THE    PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

abdominal  muscles.  The  lungs  are  filled,  "the  breath  is  held" 
(forcing  down  and  holding  the  diaphragm),  and  the  abdominal 
muscles  likewise  contract  powerfully  to  compress  the  viscera  and 
force  the  feces  into  the  rectum.  Pressure  on  the  afferent  nerves 
of  the  rectum  probably  sets  up  the  desire  to  defecate,  and  the 
contraction  of  its  walls,  as  well  as  the  relaxation  of  the  internal 
sphincter  is  a  reflex  act.  The  center  is  in  the  lower  segment 
of  the  cord,  but  it  is  connected  with  the  cerebrum,  as  is  shown 
by  emotional  influences  on  the  act. 

The  average  time  occupied  in  the  passage  of  the  residue  of  an 
ordinary  meal  from  the  mouth  to  the  rectum  is  about  24  hours. 
Something  like  12  hours  of  this  is  thought  to  be  spent  in  the 
large  intestine. 

While  it  has  been  endeavored  to  establish  clearly  the  separate 
action  of  each  fluid  with  which  the  aliment  comes  in  contact,  it 
is  to  be  remembered  that  they  form  a  mixture,  the  combined 
activity  of  whose  component  parts  results  in  the  extraction  of  all 
the  nutritive  material  from  the  bolus  in  its  long  journey  through 
the  gastro-intestinal  tract.  It  can  hardly  be  said  to  be  still  at 
any  time  during  that  passage,  the  continual  peristalsis  to  which 
it  is  subjected  facilitating  both  the  chemical  action  of  the  enzymes 
and  the  physical  phenomenon  of  absorption. 

ABSORPTION  IN  GENERAL. 

Obviously  digested  materials  are  of  no  service  in  the  vital 
economy  until  they  are  absorbed — first  by  the  circulation  and 
then  by  the  tissues  themselves.  Here  we  will  consider  only 
their  absorption  from  the  alimentary  canal,  which  process,  in 
contradistinction  to  the  other,  may  be  termed  external  absorption. 

While  it  is  known  that  the  laws  of  diffusion  and  osmosis  out- 
side the  body  are  largely  responsible  for  absorption  within  the 
organism  there  are  many  phenomena  in  connection  with  that 
process  which  cannot  be  explained  under  these  laws,  and  which 


ABSORPTION   IN    GENERAL  123 

are  indeed,  in  some  cases,  at  variance  with  them.  The  only 
explanation  at  present  to  be  offered  of  anomalous  action  is  to 
refer  it  to  some  peculiar  property  inherent  in  the  cells  them- 
selves— the  epithelium  in  case  of  the  alimentary  canal.  So 
profoundly  important  in  connection  with  physiological  activity 
are  the  laws  of  osmosis  outside  of  the  body,  and  what  is  known 
concerning  the  mutability  of  those  laws  inside  the  body,  that  a 
brief  consideration  of  the  subject  seems  necessary  to  an  intelli- 
gent conception  of  many  vital  phenomena. 

Osmosis. — When  two  different  kinds  of  gases  are  brought  in 
contact  they  mingle  with  each  other,  making  a  homogeneous 
mixture.  This  is  due  to  the  continual  motion  of  their  molecules. 
When  two  different  kinds  of  liquids  are  brought  in  contact,  a 
homogeneous  mixture  results  for  the  same  reason — unless  the 
liquids  be  non-miscible,  as  oil  and  water.  If  now  the  liquids 
happen  to  be  separated  by  a  membrane  permeable  by  both,  the 
result,  while  it  may  be  delayed,  will  be  the  same.  If,  further, 
these  liquids  hold  in  solution  substances  the  molecules  of  which 
can  penetrate  the  interposed  membrane,  there  will  likewise  be 
an  interchange  of  these  substances,  and  the  fluids  on  both  sides 
will  come  ultimately  to  have  the  same  composition.  This  pas- 
sage of  liquids  and  dissolved  matters  through  an  animal  mem- 
brane is  known  as  osmosis. 

It  must  be  remembered  that  in  the  body  particularly  the  inter- 
posed membrane  may  be  permeable  to  the  solvent,  water,  and  less 
so,  or  not  at  all,  to  the  dissolved  substances.  Materials  which 
will  in  solution  pass  through  a  membrane  are  called  crystalloids; 
those  which  will  not,  colloids.  If  simple  water  be  on  both  sides 
of  the  membrane,  the  interchange  continues  because  of  incessant 
molecular  motion;  but  the  currents  equalize  each  other,  and  no 
alteration  in  volume  or  composition  becomes  apparent.  B  ut  if  to 
the  water  on  one  side  there  be  added  a  solution  of  some  crystal- 
loid, as  sugar,  the  excess  of  water  will  pass  to  that  side.  The 
crystalloid  in  solution  is  said  to  exert  an  osmotic  pressure,  and 


124         THE   PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

that  pressure  depends  upon  the  density  of  the  solution.  In 
course  of  time,  however,  the  crystalloid  passing  itself  through  the 
membrane,  conditions  of  equal  volume  and  density  will  be  estab- 
lished on  the  two  sides  of  the  membrane,  and  osmosis  in  either 
direction  will  cease  to  be  apparent.  But  if  the  membrane  be  non- 
permeable  to  the  dissolved  substance,  an  excess  of  water  will  pass 
to  the  colloid  side  and  will  continue  so  to  pass  until  finally  it  will 
be  inhibited  by  hydrostatic  pressure  on  that  side.  This  is  taken 
as  the  measure  of  osmotic  pressure  for  the  colloid. 

All  substances  in  solution,  whether  crystalloids  or  colloids, 
exert  a  certain  osmotic  pressure;  that  is,  they  may  be  said  to 
interfere  with  the  passage  of  a  current  from  their  side  of  the 
membrane,  and  that  interference  depends  on  the  number  of 
molecules  in  solution,  or,  in  other  words,  upon  the  density  Oi 
the  fluid.  A  fanciful  but  striking  illustration  refers  the  explana- 
tion to  the  continual  molecular  motion:  the  molecules  of  the 
dissolved  substance  act  as  a  screen  to  protect  the  membrane 
from  the  water  molecules,  which  are  incessantly  moving  against 
it,  and  consequently,  in  a  given  time,  more  molecules  of  water 
will  strike  and  pass  through  the  membrane  on  the  unscreened 
than  upon  the  partially  screened  side.  Evidently  the  number  of 
molecules  in  solution  (the  density)  has  a  material  influence  upon 
the  escape  of  water  from  that  side.  Of  course,  since  a  crystal- 
loid finally  passes  to  the  less  dense  side  in  sufficient  quantity  to 
establish  an  equilibrium,  the  effect  of  its  osmotic  pressure  is  only 
temporary;  but  while  the  osmotic  pressure  of  a  colloid  may  be 
less  than  that  of  a  crystalloid,  its  effect  is  inclined  to  be  perma- 
nent. For  instance,  if  a  hypertonic  solution  (one  whose  density 
is  greater  than  that  of  blood  serum)  of  sodium  chloride  be 
injected  into  the  blood,  the  first  effect  is  to  cause  an  increased 
flow  of  water  to  the  vessels,  but  soon  enough  sodium  chloride 
passes  out  by  osmosis  to  raise  the  density  of  the  extravascular 
fluids,  and  thus  to  cause  an  escape  of  water/row  the  vessels.  On 
the  other  hand,  the  osmotic  pressure  exerted  by  the  proteids  of 


ABSORPTION   IN    GENERAL  125 

the  blood  is  comparatively  small.  But  since  they  are  here 
chiefly  as  colloids  and  tend  to  maintain  the  concentration  of  the 
circulating  fluid,  their  effect  is  a  permanent  factor  influencing 
absorption  into  the  blood-vessels. 

Isotonic  and  hypotonic  solutions  are  those  having  equal  and 
less  densities  respectively  as  compared  to  blood  serum.  Hypo- 
tonic  solutions  are  most  easily  absorbed;  hypertonic  least  easily. 
Application  of  these  principles  explains  the  rationale  of  giving 
some  medicines  in  dilute  and  others  in  concentrated  form.  As 
to  the  direction  of  the  current,  the  one  of  greater  volume  may 
be  called  the  endosmotic  and  the  one  of  lesser  volume  the  exos- 
molic.  For  example,  the  current  in  ordinary  absorption  from 
the  alimentary  canal  is  usually  termed  endosmotic,  though  it 
may  be  reversed,  as  when  magnesium  sulphate  is  given. 

When  it  is  said  that  the  greater  current  is  from  the  less  dense 
to  the  more  dense  fluid,  no  reference  is  had  to  the  direction  of 
the  solids  in  solution.  If  there  be  only  one  solid  concerned,  it 
will  be  the  one  responsible  for  the  difference  in  density  and  if  it 
be  a  crystalloid,  it  will  pass  through  the  numbrane  until  the 
density  on  the  two  sides  is  equal,  and  its  direction  will  be  opposite 
to  that  of  the  water.  If  on  the  side  of  less  density  there  be 
another  crystalloid  in  solution,  but  in  less  quantity  than  the  solid 
on  the  side  of  greater  density,  it  will  pass  in  the  direction  of  the 
greater  current  of  water  until  conditions  of  equal  concentration 
with  respect  to  this  solid  are  established.  In  the  laboratory 
the  final  result  in  any  case  of  dissolved  crystalloid  or  crystalloids 
is  two  liquids  absolutely  identical  in  composition.  A  rectal 
enema,  hypertonic  with  sodium  chloride,  will  give  up  sodium 
chloride  to  the  blood,  but  it  may  at  the  same  time  draw  upon  that 
fluid  for  urea,  for  example.  This  is  suggestive  when  an  attempt 
is  made  to  explain  the  products  of  glandular  secretion,  excretion, 
etc.  It  may  be  that  the  capillary  walls  are  permeable  to  certain 
substances  in  certain  situations  and  not  in  others. 

In  the  body  it  may  be  said  that  well-nigh  all  the  vital  functions 


126         THE   PHYSIOLOGY   OF   DIGESTION  AND   ABSORPTION 

are  dependent  upon  osmosis.  There  are  fluids  separated  by 
animal  membranes  everywhere.  In  the  alimentary  canal,  for 
instance,  is  a  fluid  containing  matters  fit  to  be  absorbed;  rami- 
fying in  the  wall  of  that  canal  are  blood  and  lymph  capillaries 
filled  with  fluid;  while  separating  the  two  is  an  animal  membrane 
consisting  of  the  alimentary  epithelium,  a  little  connective  tissue 
and  the  endothelial  lining  of  the  capillaries.  These  are  condi- 
tions most  favorable  for  osmosis,  but  the  osmotic  laws  of  the 
laboratory  are  by  no  means  immutable  in  the  body. 

From  what  has  been  said  of  osmosis  in  general,  and  consider- 
ing variations  due  to  conditions  of  circulation,  etc.,  the  following 
facts  seem  clear  as  to  absorption  in  the  body:  (i)  The  substance 
must  be  in  a  liquid  or  gaesous  state;  (2)  it  must  be  diffusible; 
(3)  the  membrane  must  be  permeable;  (4)  the  greater  current 
is  toward  the  more  dense  solution;  (5)  the  less  dense  the  solution 
the  more  quickly  will  it  be  absorbed;  (6)  the  greater  the  pressure 
in  the  vessels  the  less  rapidly  will  absorption  into  them  take 
place;  (7)  absorption  is  more  rapid  the  more  rapid  the  blood 
current  (continually  preventing  "saturation"  of  the  adjacent 
blood) ;  (8)  the  higher  the  temperature  the  more  rapid  is  absorp- 
tion; (9)  the  "vital  condition"  of  the  cells  is  the  most  important 
factor  of  all. 

A  thorough  grasp  of  these  principles  and  probabilities  will  do 
much  to  clarify  almost  all  the  phenomena  of  vital  activity,  and 
many  questions  of  a  pathological  nature. 

Absorption  from  the  Alimentary  Canal. 

It  has  been  said  that  all  digested  materials  must  find  their  way 
into  the  blood.  It  is  to  be  remembered  that  there  are  two  ways 
by  which  they  reach  the  vascular  circulation;  first,  by  direct 
absorption  into  the  capillaries  of  this  system,  and  second, 
indirectly,  by  absorption  into  the  lymphatic  circulation  and 
passage  thence  to  the  left  subclavian  vein.  Those  lymph 


ABSORPTION    FROM    THE   ALIMENTARY    CANAL  127 

capillaries  which  are  concerned  in  this  absorption  occupy  the 
villi,  and  are  called  lacteals. 

(A)  From  the  Stomach. — Since  all  classes  of  food  except  fats 
have  been  partly  digested  in  the  stomach,  it  follows  that  all 
except  fats  may  be  absorbed  here.     However,  as  a  matter  of 
observation,  the  stomach  is  of  much  less  importance  in  absorp- 
tion than  was  once  thought.     Practically,  it  is  found  that  water 
and  salts  are  passed  quickly  on  toward  the  duodenum  and  are 
not  largely  absorbed  in  the  stomach.     Sugar  and  peptones  are 
also  found  to  be   absorbed   rather  sparingly  here.     All  these 
substances  can  undoubtedly  be  absorbed  by  the  gastric  mucous 
membrane,  and  their  complete  absorption  is  prevented  only  by 
their  removal  through  the  pylorus.     It  is  interesting  to  note  that 
alcohol    and    condiments,    like    pepper    and   mustard,    greatly 
hasten  absorption,  either  by  increasing  the  blood  flow  or  by 
directly  stimulating  the  " vital  activity"  of  the  epithelium. 

(B)  From  the  Small  Intestine. — Here  absorption  of  all  classes 
of  food  is  possible,  and  here  in  fact  most  of  the  foods  are  ab- 
sorbed.    The  digestive  influences  are  more  active  upon  all  the 
aliments,  the  mucous  membrane  is  well  adapted  to  absorption 
by  reason  of  its  valvulae  conniventes  and  its  villi,  and  the  food 
necessarily  remains  in  the  small  intestine  for  a  consideraole  time. 
The  fats  are  absorbed  in  the  upper  part  of  the  small  intestine; 
for  they  pass  into  the  lacteals  of  the  villi,  and  these  do  not  exist 
in  the  lower  ileum.     The  fluids  swallowed  are  almost  com- 
pletely absorbed  here,  but  their  place  is  taken  by  the  intestinal 
secretions.     The  proteids  are  absorbed  to  the  extent  of  85  per 
cent.,  more  or  less,  before  reaching  the  large  intestine,  and  the 
carbohydrates  almost  entirely  disappear. 

(C)  From  the  Large  Intestines. — The  absorption  process  in 
the  large  intestine  is  quite  active.     The  passage  of  the  mass 
through  it  is  slower,  and  even  occupies  an  absolutely  greater 
time  than  the  journey  through  the  much  longer  small  intestine. 
The  consistence  of  the  contents  progressively  increases  owing  to 


128         THE    PHYSIOLOGY    OF    DIGESTION  AND   ABSORPTION 

continual  absorption  of  the  fluid  portions,  until  the  pultaceous 
mass  received  by  the  cecum  becomes  almost  solid  in  the  sigmoid. 
The  degree  of  consistence  may  be  said  to  be  greater  the  longer 
the  sojourn  in  the  large  intestine.  The  proteids  and  carbohy- 
drates which  have  escaped  absorption  in  the  small  intestine  are 
disposed  of  here,  partly  by  bacterial  decomposition,  and  do  not 
appear  as  such  in  the  feces.  The  absorption  of  easily  digestible 
substances  in  solutions,  such  as  eggs,  etc.,  from  the  lower  bowel, 
although  there  is  no  digestive  enzyme  there,  is  a  matter  of  com- 
mon observation,  but  one  which  lacks  explanation. 

Forms  in  Which  the  Different  Classes  are  Absorbed,  i. 
Water  and  Salts. — Of  course,  water  is  absorbed  in  connection 
with  all  the  foods  as  a  vehicle  for  them,  but  water  and  salts  as 
such  have  been  shown  to  be  absorbed  sparingly  in  the  stomach. 
They  are  soon  conveyed  to  the  small  intestine,  where  their  rapid 
disappearance  ensues.  However,  they  may  be  absorbed  any- 
where in  the  alimentary  canal.  The  loss  of  the  water  from  the 
alimentary  mass  in  the  upper  small  intestine  is  compensated  for 
by  the  secretions,  so  that  the  fluidity  of  the  contents  is  not  mate- 
rially affected  until  the  colon  is  reached.  Here  absorption  of 
water  is  active,  and  the  mass  becomes  more  and  more  solid  as  the 
rectum  is  approached. 

2.  Proteids. — It  is  agreed  that  the  first  object  of  proteid  diges- 
tion is  to  render  the  nitrogenous  foods  more  diffusible.  It  is 
also  agreed  that  the  end  products  of  such  digestion,  so  far  as 
alimentary  absorption  is  concerned,  are  proteoses  and  peptones; 
and  the  natural  conclusion,  supported  by  experimental  evidence, 
is  that  these  represent  the  forms  in  which  the  proteids  are  ab- 
sorbed. True,  leucin,  tyrosin,  etc.,  further  end  products  of 
proteolysis,  are  formed,  but  these  cannot  be  absorbed.  The 
opinion  that  proteoses  and  peptones  are  the  absorbable  forms  of 
proteids  is  correct,  for  by  far  the  largest  part  of  these  foods  are 
absorbed  in  this  shape.  It  is  supposed  also  that  syntonin  at 
least  can  itself  be  sparingly  absorbed  from  the  alimentary  canal, 


ABSORPTION    FROM    THE   ALIMENTARY   CANAL  1 29 

while  the  phenomena  of  rectal  absorption  would  point  to  the 
conclusion  that  proteid  absorption  in  other  shapes  is  possible. 
Practically,  however,  proteoses  and  peptones  may  be  regarded 
as  the  products  of  proteid  digestion,  and  their  production  as  the 
object  of  proteolysis. 

But,  although  these  substances  are  absorbed  by  the  blood- 
vessels, the  artificial  injection  of  them  into  the  veins  occasions 
untoward  effects,  or  at  least  their  rejection  through  the  organs  of 
excretion.  Furthermore,  proteoses  and  peptones  cannot  be 
detected  in  the  blood  during  alimentary  absorption.  It  follows, 
then,  that  in  their  passage  from  the  alimentary  canal  to  the  blood 
they  undergo  some  change  whereby  they  lose  their  identity  and  are 
no  longer  recognizable  as  such.  It  is  claimed  that  they  are  con- 
verted into  serum-albumin,  and  this  is  probably  true.  One 
effect  at  least  of  the  change  is  that  they  are  now  (in  the  blood) 
less  diffusible,  more  complex,  and  consequently  remain  more 
easily  a  constituent  part  of  that  fluid. 

The  proteids  enter  the  radicles  of  the  portal  vein. 

3.  Carbohydrates. — The  sugar  of  the  blood  is  dextrose,  and 
if  cane  sugar  be  introduced  into  the  veins  it  is  rejected  by  the 
urine  without  being  changed.     It  may  be  said  that,  with  a  few 
exceptions,  all  the  carbohydrates  are  converted  into  dextrose  or 
dextrose  and  levulose,  before  entering  the  blood.     This  form  of 
sugar  is  easily  oxidized  in  the  tissues.     It  is  conveyed  directly 
to  the  liver  by  the  portal  vein. 

4.  Fats. — The  digestive  end  of  the  fats  has  been  seen  to  be 
emulsions  and  soaps.     They  pass  into  the  intestinal  lymphatics, 
or  lacteals.     Their  absorption  is  a  mechanical  process.     They 
enter  and  pass  through  the  epithelial  cells  and  basement  mem- 
brane of  the  villus.     Having  thus  passed  into  the  stroma  of  the 
villus,  their  entrance  into  the  lacteal  is  easy;  for  undoubtedly 
lymph  spaces  in  the  stroma  are  connected  with  the  stomata  of 
the  central  lymph  capillary,  and  there  is  a  more  or  less  constant 
flow  of  lymph  through  these  spaces  toward  the  lacteal.     The 

9 


130         THE    PHYSIOLOGY    OF    DIGESTION   AND   ABSORPTION 

tendency,  therefore,  of  the  fats  to  enter  the  lacteal  is  physically 
natural.  It  is  a  curious  fact  that  the  peptones  and  sugars, 
having  penetrated  the  lining  epithelium  of  the  villus,  enter  the 
blood  instead  of  the  lymph  capillaries. 

A  number  of  circumstances,  such  as  the  rate  of  absorption, 
the  persistent  direction  of  the  current  toward  the  blood  in  the 
face  of  superior  pressure,  the  disappearance  of  non-osmotic 
substances  from  the  canal,  etc.,  are  frequently  at  variance  with 
laboratory  experiments.  Application  of  the  laws  of  osmosis  to 
the  vital  processes  is  seemingly  subject  to  many  variations,  and 
explanation  of  many  of  the  phenomena  of  absorption  in  the 
body  waits  upon  a  clearer  understanding  of  the  so-called  "vital 
activity"  of  the  tissues. 


CHAPTER  VIII. 
RESPIRATION. 

Object. — The  object  of  respiration  is  to  furnish  oxygen  to 
the  tissues  and  remove  carbon  dioxide  from  them.  The  inter- 
vention of  the  lungs  and  blood  is  necessary  to  accomplish  this 
end.  At  each  inspiration  a  certain  volume  of  air  is  taken  into 
the  lungs,  and  from  it,  while  in  these  organs,  is  removed  a  cer- 
tain amount  of  oxygen  which  enters  the  blood  of  the  pulmonary 
capillaries.  At  each  expiration  there  is  removed  from  the  lungs 
a  certain  volume  of  air,  and  it  contains  a  proportion  of  carbon 
dioxide  over  and  above  that  contained  in  the  ordinary  atmosphere, 
i.  e.,  in  the  inspired  air;  this  carbon  dioxide  is  removed  from  the 
blood  of  the  pulmonary  capillaries  and  enters  the  air  in  the  lungs. 
The  entrance  and  exit  of  air  to  and  from  the  lungs,  in  obedience 
to  movements  to  be  noticed  later,  constitutes  what  is  commonly 
called  respiration ;  but  the  mere  tide  of  the  air  inward  and  out- 
ward is  of  no  significance  unless  the  interchange  of  oxygen  and 
carbon  dioxide  takes  place. 

Internal  Respiration. — Nor  is  this  interchange  of  value  unless 
another  occurs  in  the  tissues.  The  oxygen  which  has  entered  the 
pulmonary  blood  is  conveyed  by  the  circulation  to  a  point  where 
the  fluid  is  brought  into  very  close  relationship  with  the  tissues 
(namely,  in  the  capillaries),  and  is  here  given  up  to  the  cells; 
furthermore,  at  the  same  place  the  cells  give  up  carbon  dioxide  to 
the  capillary  blood.  It  is  only  for  the  purpose  of  effecting  this 
last  interchange  that  there  is  any  respiration,  or  any  respiratory 
apparatus.  Inspiration  and  expiration,  the  pulmonary  inter- 
change of  gases,  the  transportation  of  oxygen  and  carbon  dioxide 
to  and  away  from  the  cells,  are  all  equally  immaterial  except  as 
being  means  to  the  accomplishment  of  this  end.  It  would  make 


132  RESPIRATION 

no  difference  whether  pulmonary  respiration  were  kept  up  or  not 
if  oxygen  could  be  introduced  into  the  blood  and  carbon  dioxide 
removed  from  it  in  some  other  equally  efficient  way.  So  far  as 
the  cell  is  dependent  on  the  acquisition  of  oxygen  and  the  re- 
moval of  carbon  dioxide,  it  would  make  no  difference  if  there 
were  no  respiration  and  no  circulation  if  these  materials  could 
be  acquired  and  removed  in  some  other  equally  efficient  way. 

On  the  other  hand,  it  were  useless  to  keep  up  artificial  respira- 
tion or  to  inject  oxygen  into  the  lungs  if  the  cells,  through  some 
disability,  cannot  take  up  the  oxygen  furnished,  or  if  the  circu- 
lation cannot  absorb  or  convey  the  oxygen. 

It  is  seen  that,  from  the  standpoint  of  the  blood,  the  interchange 
of  gases  in  the  lungs  is  exactly  opposite  to  that  in  the  tissues; 
that  is  to  say,  in  the  lungs  it  loses  carbon  dioxide  and  gains  oxygen, 
while  in  the  tissues  it  loses  oxygen  and  gains  carbon  dioxide. 
The  pulmonary  interchange  is  properly  termed  external  res- 
piration in  contradistinction  to  that  in  the  tissues  which  is 
termed  internal  respiration. 

It  is  needless  to  comment  upon  the  universal  necessity  of  oxy- 
gen to  the  life  of  cells.  Its  appropriation  is  to  be  looked  upon 
as  a  part  of  the  nutritive  process;  and,  indeed,  while  in  the  long 
run,  cells  are  certainly  dependent  upon  the  nutriment  furnished 
by  the  ordinary  aliments,  they  will  retain  their  vital  activity  for 
a  longer  time  when  deprived  of  any  or  all  of  these  than  when 
deprived  of  oxygen  alone.  This  gas  is  more  immediately 
necessary  to  the  maintenance  of  life  than  is  any  other  substance. 

Since,  in  order  to  bring  about  internal  respiration  in  the  human 
being,  the  lungs  and  circulation  happen  to  be  necessary,  attention 
will  have  to  be  directed  to  the  respiratory  phenomena  taking 
place  in  both. 

ANATOMY  OF  THE  RESPIRATORY  ORGANS. 

It  will  be  considered  that  the  air  has  passed  through  the  pos- 
terior nares  into  the  pharynx  and  is  ready  to  enter  the  larynx. 


ANATOMY    OF    THE    RESPIRATORY    ORGANS 


133 


The  Larynx. — This  lies  in  front  of  the  esophagus,  its  upper 
opening  communicating  with  the  middle  pharynx.  It  is  com- 
posed of  four  cartilages  and  the  muscles  and  ligaments  which 
hold  them  together.  The  cartilages  keep  its  lumen  constantly 
open,  while  the  muscles  effect  movements  concerned  in  degluti- 


FIG.  47. — Diagram  of  the  respiratory  organs. 

The  windpipe  leading  down  from  the  larynx  is  seen  to  branch  into  two  large 
bronchi,  which  subdivide  after  they  enter  their  respective  lungs.      (Yeo.) 


tion,  respiration  and  phonation.  The  cartilages  are  the  thyroid, 
cricoid  and  two  arytenoids.  The  two  alae  of  the  thyroid  meet  at 
an  acute  angle  in  front  to  form  the  Adam's  apple.  The  cricoid 
is  at  the  lower  end  of  the  larynx,  completely  surrounding 
it.  The  arytenoids  are  movable  and  rest  upon  the  back  of  the 
cricoid.  (Fig.  48.) 

The  vocal  cords,  two  ligamentous  bands  covered  by  a  thin 
layer  of  mucous  membrane,  stretch  antero-posteriorly  across 
the  upper  end  of  the  larynx,  while  the  false  vocal  cords,  having 
nothing  to  do  with  phonation,  and  pinker  in  color,  are  above 


134 


RESPIRATION 


FIG.  48. — Outline  showing  the  general  form  of  the  larynx,  trachea,  and 
bronchi,  as  seen  from  behind. 

h,  great  cornu  of  the  hyoid  bone;  t,  superior,  and  t',  the  inferior  cornu  of  the 
thyroid  cartilage;  e,  epiglottis;  a,  points  to  the  back  of  both  the  arytenoid  cartilages, 
which  are  surmounted  by  the  cornicula;  c,  the  middle  ridge  on  the  back  of  the 
cricoid  cartilage;  tr,  the  posterior  membranous  part  of  the  trachea;  b,  b',  right  and 
left  bronchi.  (Kirkes  after  Allen  Thomson.) 


THE   TRACHEA  135 

and  parallel  with  the  true  cords.  A  small  triangular  leaflet  of 
fibre-cartilage  is  attached  by  its  base  to  the  base  of  the  tongue 
and  to  the  upper  anterior  part  of  the  larynx.  This  is  the  epi- 
glottis. It  fits  accurately  over  the  opening  of  the  larynx,  and 
during  the  act  of  deglutition  is  closed  to  prevent  the  entrance  of 
food,  saliva,  etc.  Except  during  deglutition  the  epiglottis  is 
raised  and  there  is  free  passage  of  air  into  and  out  of  the  laryn- 
geal  cavity.  The  vocal  cords  are  fixed  anteriorly  to  a  point 
between  the  alae  of  the  thyroid  and  posteriorly  to  the  movable 
arytenoids.  Intrinsic  muscles  have  the  power  of  so  moving 
the  arytenoids  as  to  separate  and  approximate  the  posterior 
attachments  of  the  cords  and  thus  increase  or  decrease  the  size 
of  the  rima  glottidis.  During  inspiration  these  muscles  act  to 
separate  the  cords  and  allow  free  entrance  of  air  into  the  trachea. 
When  this  act  has  ceased  they  relax  and  the  cords  are  passively 
approximated.  The  expiratory  act  separates  the  cords  and  they 
afford  no  obstruction  to  the  exit  of  air.  The  inspiratory  act,  on 
the  other  hand,  tends  to  draw  the  cords  together  and  the  active 
intervention  of  the  muscles  is  necessary  to  keep  the  glottis  open. 
The  Trachea.— The  trachea  succeeds  the  larynx  in  the  respi- 
ratory tract.  It  begins  at  the  cricoid  cartilage  and  extends  down- 
ward for  about  four  and  a  half  inches  where  it  bifurcates  to  form 
the  right  and  left  bronchi,  one  of  which  goes  to  each  lung.  The 
trachea  consists  of  an  external  fibrous  membrane,  between  the 
layers  of  which  are  a  number  of  cartilaginous  rings,  and  an  inter- 
nal mucous  membrane.  The  rings  are  the  most  striking  part  of 
the  trachea.  They  serve  to  keep  the  canal  open  at  all  times. 
The  inspiratory  effort  would  otherwise  collapse  the  walls  and 
prevent  the  entrance  of  air.  These  rings  are  sixteen  to  twenty 
in  number,  and  are  lacking  in  the  posterior  third  or  fourth  of  the 
circumference.  They  are,  therefore,  not  true  rings.  The  inter- 
val between  their  ends  is  filled  with  fibrous  and  non-striped  mus- 
cular tissue.  The  mucous  membrane  is  lined  by  ciliated  epithe- 
lium, and  has  mucous  glands  in  its  substance  (Figs.  47,  48). 


136  RESPIRATION 

The  Bronchi. — The  primitive  bronchi  are  of  the  same  essen- 
tial structure  as  the  trachea.  The  right  is  the  larger,  shorter, 
and  more  nearly  horizontal.  This  probably  accounts  for  the 
more  frequent  lesions  in  the  right  lung.  Penetrating  the  lung 
substance  they  divide  and  subdivide  until  each,  by  its  ramifica- 
tions, communicates  with  every  air  vesicle  in  that  lung.  When 
the  primitive  bronchus  has  divided,  the  incomplete  cartilaginous 


Bronchial  Muscfe. 


Bronchia/  flrtery. 


G fa  not  acini  fe  of  net 

FIG.  49. 

T.  S.,  intra-pulmonary  bronchus  of  cat.     PA  and  PV,  pulmonary  artery  and  vein; 
bv,  bronchial  vein;  V,  air  vesicles.     (Stirling.) 


rings  are  replaced  by  irregular  plates  of  cartilage,  which  are  so 
arranged  as  to  completely  encircle  the  tube.  These  extend  as 
far  as  the  division  of  the  tubes  into  branches  -^  in.  in  diameter. 

Surrounding  the  tubes  in  the  lung  substance  is  a  circular  layer 
of  plain  muscular  fibers;  these  cease  only  at  the  air  vesicles. 
Elastic  fibrous  tissue  is  also  present  everywhere  in  the  bronchial 
walls  and  is  continued  over  the  vesicles  themselves. 

Bronchial  tubes  above  -     in.  in  diameter  have  in  their  walls 


AIR    VESICLES  137 

cartilaginous  plates,  muscular  tissue,  fibrous  elastic  and  inelastic 
tissue  and  a  lining  membrane  of  ciliated  epithelium. 

Bronchial  tubes  -^  in.  in  diameter,  and  smaller,  have  in  their 
walls  the  same  elements  excepting  the  cartilage;  but  as  the  tubes 
subdivide  their  walls  grow  continuously  thinner,  and  the  inelas- 
tic tissue  become  less  and  less  in  amount,  until  it  finally  prac- 
tically disappears;  the  ciliated  epithelial  cells  gradually  give 
place  to  a  single  layer  of  squamous  cells  in  the  smallest  tubes. 
The  smallest  bronchial  tubes,  the  bronchioles,  are  from  T^j-  to 
-fa  in.  in  diameter.  Of  course  everywhere  in  the  walls  there 
are  vessels  and  nerves. 

The  Air  Vesicles. — Each  bronchiole  opens  into  a  collection 
of  air  vesicles,  or  cells,  called  a  pulmonary  lobule.  The  term 
lobulette  will  be  here  applied  to  it,  however,  reserving. the  word 
lobule  for  a  collection  of  lobulettes  about  J  in.  in  diameter.  The 
bronchiole  entering  the  lobulette  becomes  the  infundibulum 
(Fig.  50),  a  slightly  dilated  canal  from  which  are  given  off  from 
eight  to  sixteen  oblong  vesicles,  the  true  air  cells.  The  cells  are 
a  little  deeper  than  they  are  wide  and  end  in  blind  extremities. 
The  diameter  of  the  lobulette  is  about  -^VrV  ^n>>  *nat  °f  the 
vesicle  about  2oo~7lo  in-  ^  nas  been  estimated  that  there  are 
some  725,000,000  of  these  vesicles  in  the  lungs  and  that  their 
combined  area  is  something  over  two  hundred  square  yards. 

The  walls  of  the  air  cells  are  very  thin,  being  composed  of  a 
single  layer  of  flattened  epithelium  together  with  highly  elastic 
fibrous  tissue.  Ramifying  in  this  latter  is  a  most  abundant  supply 
of  capillaries,  which  are  larger  here  than  anywhere  else  in  the 
body.  The  physical  conditions  are  most  favorable  for  the  ex- 
change of  gases  between  the  blood  and  air,  each  capillary  being 
exposed  to  vesicles  on  both  sides,  and  the  air  and  blood  being 
separated  only  by  the  very  thin  walls  of  the  capillary  and  vesicle. 
The  elastic  tissue  is  very  important  in  expelling  the  air  from  the 
cells  when  the  inspiratory  effort  has  ceased. 

For  the  nutrition  of  the  bronchi  and  lung  substance  arterial 


138  RESPIRATION 

blood  is  furnished  by  the  bronchial  artery,  which  enters  and 
ramifies  with  the  bronchi.  The  entire  mass  of  venous  blood 
passes  directly  from  the  heart  through  the  pulmonary  artery  to 
the  lungs  to  be  arterialized,  and  it  is  the  capillaries  of  this  artery 
which  furnish  the  abundant  network  between  the  air  cells. 


Pig.  50 — Terminal  branch  of  a  bronchial  tube,  with  its  infundibula  and  air- 
sacs,  from  the  margin  of  the  lung  of  a  monkey,  injected  with  quicksilver. 

a,  terminal  bronchial  twig;  b  b,  air-sacs,  c  c,  infundibula.     Xio.  (Kirkes  after  E.  E. 
Schulze.) 

The  lungs  have  the  shape  of  irregular  cones,  their  bases  rest- 
ing on  the  diaphragm  and  their  apices  extending  to  points  a 
little  above  the  clavicles.  They  are  completely  separated  from 
each  other  by  the  mediastinum  and  their  external  surfaces  are 
covered  by  the  pleura,  a  serous  membrane  similar  to  the  peri- 
toneum and  reflected  from  the  thoracic  wall.  The  right  lung  is 
divided  by  fissures  into  three  lobes  and  the  left  into  two.  Super- 
ficially the  lung  substance  is  seen  to  be  subdivided  into  areas 
about  J  in.  in  diameter  called  the  lobules.  Each  lobule  is  com- 
posed of  a  number  of  lobulettes  as  above  mentioned. 

MECHANISM  OF  RESPIRATION. 

Respiration  implies  the  more  or  less  regular  entrance  and  exit 
of  air  to  and  from  the  lungs.  The  entrance  is  inspiration; 
the  exit  expiration.  Now,  the  thorax  is  a  closed  cavity,  not- 


MECHANISM    OF   RESPIRATION  139 

withstanding  the  fact  that  the  lungs  have  an  opening  (the  trachea) 
by  which  they  communicate  with  the  external  air;  and,  so  far  as 
the  simple  ingress  and  egress  of  air  is  concerned,  the  question  of 
pulmonary  respiration  resolves  itself  into  one  of  pure  mechanics. 
The  lungs  may  be  looked  upon  as  a  bag  (or  two  bags)  in 
the  thoracic  cavity.  Inspired  air  does  not  enter  the  thoracic 
cavity,  but  this  bag  which  is  in  it.  This  fact  is  of  the  greatest 
importance. 

Furthermore,  the  lungs  are  everywhere  in  contact  with  the 
thoracic  wall  by  their  pleural  surfaces.  They  are  composed 
very  largely  of  highly  developed  elastic  tissue,  but  are  perfectly 
passive  themselves.  That  is  to  say,  they  possess  no  power  of 
expansion  except  in  obedience  to  extraneous  influences.  As 
found  in  the  thorax  they  possess  a  contractile  power,  but  only 
because  certain  forces  have  put  their  elastic  tissue  on  the  stretch, 
and  the  contraction  is  a  simple  effort  of  the  tissue  to  return  to  the 
condition  which  characterized  it  before  it  was  subjected  to  the 
expanding  force. 

Before  birth  there  is  no  air  in  the  lungs,  and  this  is  the  only 
time  when  the  elastic  tissue  is  not  on  the  stretch.  The  bronchi- 
oles and  air  cells  are  collapsed,  but  the  thorax  is  contracted  and 
the  pulmonary  and  thoracic  walls  are  in  contact  by  their  respec- 
tive pleural  surfaces.  When  the  child  is  born  an  inspiration 
fills  the  lungs  and  they  are  never  thereafter  devoid  of  air.  They 
collapse  to  a  certain  extent  and  leave  the  thoracic  wall  when  the 
chest  is  opened,  but  cannot  empty  themselves  entirely  because  the 
walls  of  the  bronchioles  collapse  before  all  the  air  can  escape. 
This  collapse  of  the  lungs  when  the  chest  wall  is  opened  shows 
that  the  lung  structure  is  in  a  constant  state  of  tension,  which 
tension  has  always  a  tendency  to  empty  the  lungs,  but  cannot  do 
so  because  the  thorax  can  contract  only  so  far,  and  when  its  con- 
traction has  reached  its  limit,  for  the  lung  to  contract  farther 
would  mean  a  separation  of  the  pulmonary  and  thoracic  walls 
and  the  formation  of  a  vacuum  between  them.  The  additional 


140  RESPIRATION 

reason  above  given,  namely  the  collapse  of  the  bronchioles  be- 
fore all  the  air  can  escape,  is  inoperative  under  normal  condi- 
tions and  need  not  be  considered. 

Causes  of  Respiratory  Movements.— Seeing  that  the  lung 
structure  has  always  a  tendency  to  empty  itself  of  air,  it  follows 
that  inspiration  cannot  be  dependent  upon  the  lung  itself. 
Granting,  from  the  physical  conditions  present,  that  the  lungs 
and  thorax  must  expand  and  contract  together,  the  expansion  of 
the  lungs  in  inspiration  is  a  consequence  and  not  a  cause  of  the 
thoracic  expansion,  and  the  contraction  of  the  lungs  in  expiration 
is  a  cause  and  not  a  consequence  of  thoracic  contraction.  This 
statement  as  to  expiration  applies  only  to  ordinary  tranquil 
respiration,  as  will  be  seen  later.  Speaking  broadly  then,  inspira- 
tion is  an  active  and  expiration  a  passive  process.  That  is, 
inspiration  occurs  as  a  result  of  the  activity  of  certain  muscles 
which  operate  to  expand  the  thorax,  and  expiration  as  a  conse- 
quence simply  of  the  cessation  of  activity  on  the  part  of  those 
muscles  and  the  passive  contraction  of  the  lung  tissue. 

The  relation  of  the  thorax  and  lungs  and  the  action  of  each  in 
respiration  may  be  illustrated.  Suppose  a  bellows,  which,  say 
for  some  mechanical  reason,  cannot  completely  collapse  and 
which  is  itself  air-tight,  to  contain  a  thin  rubber  bag  commu- 
nicating by  a  tube  with  the  external  air;  suppose  the  bag  con- 
forms in  general  outline  to  the  shape  of  the  bellows,  and  under 
a  moderate  degree  of  distention  completely  fills  the  cavity  of  the 
bellows  when  the  latter  is  collapsed  as  far  as  possible.  Now,  it 
being  understood  that  the  bag  was  somewhat  distended  to  cause 
it  to  fill  the  bellows,  and  that  all  air  has  been  allowed  to  escape 
by  a  temporary  opening  from  between  the  walls  of  the  two  and 
the  bellows  itself  made  air-tight  afterwards,  it  follows  that  unless 
the  bellows  can  contract  the  bag  will  remain  distended  and  will 
not  leave  the  bellows  wall,  although  it  will  have  a  constant  tend- 
ency to  do  so.  It  is  also  apparent  that,  since  the  bag  exerts  a 
continual  compressing  effect  on  its  contents,  the  pressure  inside 


INSPIRATION  141 

it  will  be  greater  than  that  outside  between  it  and  the  bellows 
wall.  Under  these  conditions  there  will  be  a  constant  tendency 
on  the  part  of  the  bellows  to  collapse,  and  some  active  force  will 
be  necessary  to  expand  it;  when  it  is  made  to  expand  the  con- 
tained bag  will  expand  with  it.  Suppose  the  expansion  to  be 
stopped  at  a  certain  point  and  the  bellows  held  (to  prevent  con- 
traction); it  is  obvious  that  now  the  pressure  inside  the  bag  is 
greater,  while  that  outside  between  its  walls  and  those  of  the  bel- 
lows is  less,  than  when  the  expansion  began;  that  is,  the  bag  has 
become  distended  more  and  is  exerting  a  greater  compressing 
effect  upon  its  contents.  If  now  the  bellows  be  simply  released, 
both  the  bag  and  the  bellows  will  contract  and  the  former  will 
empty  itself  so  far  as  the  latter  will  allow;  but  when  the  bellows 
has  reached  the  limit  of  its  contraction  the  bag  also  ceases  to 
contract,  although  it  remains  in  a  constant  state  of  tension.  If 
at  any  time  air  be  admitted  to  the  bellows  proper  the  bag  will  at 
once  collapse. 

This  illustration  can  be  applied  to  the  mechanical  principles 
obtaining  in  ordinary  respiration.  The  bellows  is  the  air-tight 
thorax  which  cannot  contract  beyond  a  certain  point;  the  rubber 
bag  is  the  elastic  lungs  under  constant  tension,  communicating 
by  the  trachea  with  the  external  air  and  following,  or  being  fol- 
lowed by,  the  movements  of  the  thorax;  the  pressure  in  the  bag 
and  between  it  and  the  bellows  wall  represents  the  mtrapulmo- 
nary  and  intrathoracic  pressures  respectively. 

It  will  be  noticed  later  that  this  illustration  does  not  go  quite 
far  enough  to  explain  a  few  of  the  phenomena  of  expiration,  but 
it  could  very  easily  be  made  to  do  so. 

Inspiration. — Any  force  which  expands  the  thorax  aids  in  in- 
spiration; and  any  muscles  which  increase  any  of  the  thoracic 
diameters  expand  the  thorax.  The  diameters  increased  are 
chiefly  the  (i)  vertical  and  (2)  antero-posterior. 

The  vertical  is  increased  by  descent  of  the  diaphragm,  which 
descent  is  caused  by  its  contraction,  since,  owing  to  the  intra- 


142  RESPIRATION 

thoracic  "pull"  exerted  upon  it,  it  is  normally  vaulted  upward. 

The  antero-posterior  diameter  is  increased  chiefly  by  the  ele- 
vation of  the  ribs.  Since  these  bones,  attached  posteriorly  to  the 
spinal  column,  run  not  only  forward  but  also  downward  to  join 
the  sternum  by  the  costal  cartilages,  it  follows  that  the  elevation 
of  their  anterior  ends  will  increase  the  diameter  in  question. 

Muscles  oj  Inspiration. — Elevation  of  the  ribs  is  effected  by  a 
number  of  muscles.  The  three  scaleni  are  attached  above  to  the 
cervical  vertebrae  and  below  to  the  first  and  second  ribs;  their 
action  elevates  not  only  these  ribs  but  the  whole  anterior  chest 
wal'. 

The  action  of  the  intercostales  externi  is  still  a  subject  of  dis- 
pute in  connection  with  the  physiology  of  respiration.  These 
muscles  are  attached  externally  to  the  adjacent  borders  of  the 
ribs,  and  thus  occupy  the  intercostal  spaces.  Their  fibers  are 
directed  downward  and  forward,  and  the  effect  of  contraction 
of  any  single  intercostal  muscle  would  be  to  approximate  the 
two  ribs  to  which  it  is  attached;  but  if  it  can  be  assumed  that  the 
first  rib  is  fixed,  then,  from  the  direction  of  their  fibers,  the  ex- 
ternal intercostals  will  render  the  ribs  more  nearly  horizontal  by 
raising  their  anterior  movable  extremities.  It  seems  that  the 
first  rib  is  prevented  from  descending,  probably  by  the  simulta- 
neous contraction  of  the  scaleni.  The  intercostales  interni  have  a 
direction  almost  at  right  angles  to  that  of  the  externi;  the  sternal 
portions  of  these  act  from  the  sternum  and  also  elevate  the 
anterior  extremities  of  the  ribs.  The  levatores  costarum  are 
attached  to  the  transverse  processes  of  the  dorsal  vertebrae  and 
to  the  upper  borders  of  the  ribs  posteriorly.  The  transverse 
processes  are  fixed  points  and  the  ribs  are  movable  on  their  spinal 
articulations.  Contraction  of  these  muscles  is,  therefore,  very 
efficient  in  elevating  the  anterior  ends  of  the  ribs. 

The  action  of  the  diaphragm  is  the  most  notable  of  the  mus- 
cular phenomena  connected  with  respiration,  and  it  deserves  to 
be  called  the  "muscle  of  respiration  " 


EXPIRATION  143 

^These  are  the  muscles  which  are  chiefly  concerned  in  ordinary 
inspiration.  Their  combined  action  also  increases  slightly 
the  transverse  diameter  of  the  chest.  But  there  are  certain 
others,  known  as  auxiliary  muscles  of  inspiration,  which  are  called 
into  play  during  profound  or  forced  inspiration.  Their  action 
is  evident  from  their  attachments — all  operating  chiefly  to  in- 
crease the  antero-posterior  diameter.  They  are  the  serratus 
posticus  superior,  sterno-mastoideus,  levator  anguli  scapula,  tra- 
pezius,  pectoralis  minor,  pectoralis  major  (costal  portion),  serratus 
magnus,  rhomboidei  and  electores  spince.  It  will  be  noticed  that 
several  of  these  which  usually  take  their  point  on  the  chest,  as, 
for  example,  the  sterno-mastoideus,  pectorales,  etc.,  must,  in 
order  to  aid  inspiration,  take  their  fixed  points  at  their  other 
extremities. 

Expiration. — When  the  force  which  expands  the  chest  during 
inspiration  ceases  to  operate,  expiration  follows.  Not  only  does 
the  elastic  (i)  lung  tissue  force  out  the  air,  but  the  (2)  thoracic 
walls,  by  their  costal  cartilages  and  their  intercostal  tissues,  are 
themselves  elastic,  and  this  elasticity,  aided  by  the  (3)  "tone"  of 
the  muscles  which  have  been  put  upon  the  stretch  during  inspira- 
tion and  which  are  now  seeking  to  return  to  their  normal  con- 
dition, tends  to  restore  the  thorax  to  the  dimensions  it  had  pre- 
vious to  the  inspiratory  act.  So  far  no  actual  muscular  con- 
traction has  been  brought  into  play,  and  it  is  here  assumed  that 
none  is  usually  concerned  in  the  expiratory  act  of  ordinary  tran- 
quil respiration. 

Some  maintain  that  the  costal  portions  of  the  intercostales 
interni  particularly  are  expiratory  in  quiet  breathing;  they  do 
contract  and  the  ribs  approach  each  other  during  the  act,  but  it 
is  probable  that  they  serve  only  to  maintain  the  proper  degree  of 
tension  of  the  intercostal  tissues. 

Although  the  elastic  reaction  of  the  lung  tissue  during  expira- 
tion operates  together  with  the  elasticity  of  the  thoracic  wall  in 
diminishing  the  antero-posterior  diameter  of  the  chest,  it  is 


144  RESPIRATION 

chiefly  effective  in  diminishing  the  vertical  diameter  by  raising 
the  diaphragm.  It  exerts  a  certain  "suction"  upon  that  muscle, 
causing  it  to  arch  upward  in  following  the  contracting  lungs. 
It  is  seen,  therefore,  that  during  inspiration  the  chest  wall  and 
diaphragm  exert  "suction"  upon  the  lungs,  causing  them  to  fol- 
low, and  during  expiration  the  lungs  exert  "suction"  upon  the 
chest  wall  and  diaphragm,  causing  them  to  follow. 

Forced  Expiration. — It  is  evident  that,  while  ordinary  expi- 
ration is  a  passive  process,  a  person  can  voluntarily  force  out  of 
his  lungs  more  air  than  is  ordinarily  expelled,  as  in  singing,  blow- 
ing, talking,  etc.  This  is  effected  by  certain  muscles  whose  con- 
traction diminishes  the  thoracic  capacity,  chiefly  by  depressing 
the  ribs  and  elevating  the  diaphragm.  Those  which  depress  the 
ribs  are  the  intercostales  interni,  infracostales  and  triangularis 
sterni.  Those  which  elevate  the  diaphragm  do  so  by  compressing 
the  abdominal  contents  and  forcing  them  up  against  that  muscle. 
They  are  the  obliquus  externus,  obliquus  internus  transversalis 
and  rectus  abdominis.  These  depress  the  chest  wall  as  well. 

Rhythm  of  Respiration. — Under  ordinary  conditions  inspi- 
ration and  expiration  follow  each  other  in  a  regular  rhythmical 
fashion.  Some  hold  that  an  interval  follows  inspiration  before 
expiration  begins,  but  this  is  probably  not  correct.  Indeed,  it 
is  doubtful  if  there  be  an  interval  following  expiration,  though 
it  will  be  here  considered  that  there  is  a  brief  one.  Expiration 
is  a  little  longer  than  inspiration.  The  inspiratory  act  is  of  uni- 
form intensity  throughout,  while  the  expiratory  act  gradually 
diminishes  in  intensity  as  it  approaches  completion — a  circum- 
stance to  be  expected  from  the  physical  conditions  causing  it. 

After  every  six  to  ten  respiratory  acts  a  more  profound  (sigh- 
ing) inspiration  than  usual  is  taken,  the  effect  being  a  more 
thorough  changing  of  the  pulmonary  contents.  Coughing, 
sneezing,  hiccoughing,  laughing,  etc.,  all  interfere  with  rhythmical 
respiration. 

Modified  Respiration. — In  coughing  and  sneezing  a  profound 


RATE    OP    RESPIRATION  145 

inspiration  precedes  a  violent  convulsive  contraction  of  the 
expiratory  muscles.  Sighing  is  an  expression  on  the  part  of  the 
tissues  that  more  oxygen  is  needed  and  that,  therefore,  the  con- 
tents of  the  lungs  must  be  more  completely  changed.  Yawning 
is  a  phenomenon  similar  to  sighing,  but  may  not  represent 
deficient  oxygenation,  as  when  it  occurs  from  contagion.  Except 
in  the  contraction  of  different  facial  muscles,  sobbing  and  laughing 
are  identical  from  a  respiratory  standpoint;  in  both  there  is  a 
succession  of  quick  contractions  of  the  diaphragm.  Hiccough 
is  an  involuntary  contraction  of  the  diaphragm  accompanied  by 
closure  of  the  glottis.  It  takes  place  during  inspiration.  In 
hawking  the  glottis  is  open  and  a  continuous  expiratory  current 
is  sent  through  the  narrowed  passage  between  the  base  of  the 
tongue  and  the  soft  palate.  Snoring  occurs  with  the  mouth  open; 
the  current  of  air  throws  the  uvula  into  vibration  and  produces 
the  characteristic  sounds. 

Sounds  of  Respiration. — When  the  ear  is  applied  to  the  chest 
there  is  heard  during  inspiration  a  breezy  expansive  sound  of 
slightly  increasing  intensity  throughout,  and  ceasing  abruptly 
at  the  end  of  the  act.  Immediately  begins  the  expiratory  sound, 
very  short,  lower  in  pitch  than  the  inspiratory,  and  gradually 
decreasing  in  intensity  until  it  is  lost  before  expiration  is  more 
than  one-fourth  finished.  When  listening  over  a  large  bronchus 
this  sound  is  prolonged  and  has  a  higher  pitch  than  usual. 
Respiratory  sounds  are  more  pronounced  in  the  female  than  in 
the  male  chest,  owing  to  the  predominance  of  costal  breathing 
in  the  former  sex. 

Rate  of  Respiration. — The  respiratory  rate  sustains  a  fairly 
constant  relation  to  the  cardiac  rate,  the  ratio  being  about  one 
to  four.  This  makes  the  average  number  of  respirations  about 
eighteen  per  minute  for  adults.  In  a  general  way  this  rate  is 
subject  to  variations  from  the  same  causes  as  that  of  the  pulse. 
Any  appreciable  fall  in  the  amount  of  oxygen  in  the  inspired  air 
will  increase  the  number  of  respirations  for  obvious  reasons. 

10 


146  RESPIRATION 

The  frequency  and  depth  usually  bear  an  inverse  ratio  to  each 
other. 

Types  of  Respiration.— (i)  Costal  respiration  is  that  carried 
on  by  the  chest  walls;  (2)  diaphragmatic,  that  effected  by  the 
diaphragm.  In  the  former  type  movements  of  the  thorax  are 
concerned;  in  the  latter,  movements  of  the  abdomen.  According 
as  the  movements  in  costal  respiration  are  more  pronounced  in 
the  upper  or  lower  segment  of  the  chest,  that  type  is  subdivided 
into  (a)  superior  costal  and  (b)  inferior  costaL 

In  young  children  the  diaphragmatic,  or  abdominal,  type 
prevails;  in  adult  males  a  combination  of  the  inferior  costal  and 
abdominal;  in  adult  females  the  superior  costal.  The  last  cir- 
cumstance is  probably  due  in  part  to  the  mode  of  dress  in  civilized 
countries,  and  in  part  to  the  provision  against  encroachment 
of  the  uterus  upon  the  abdominal  cavity  during  pregnancy. 

Intrapulmonary  and  Intrathoracic  Pressure. — It  is  evi- 
dent that  during  inspiration  the  pressure  inside  the  lungs  (intra- 
pulmonary)  is  less  than  the  ordinary  atmospheric  pressure;  this, 
in  fact,  is  the  immediate  cause  of  the  entrance  of  air.  It  is  also 
evident  that  during  expiration  the  intrapulmonary  pressure, 
owing  to  the  compressing  effect  of  the  lung  tissue  and  the  tho- 
racic walls,  is  greater  than  the  outside  atmospheric  pressure; 
this  is  the  immediate  cause  of  the  exit  of  air.  In  both  acts  the 
air  rushes  in  or  out,  as  the  case  may  be,  in  an  effort  to  maintain 
the  same  pressure  inside  the  lungs  as  exists  in  the  surrounding 
atmosphere.  It  is  convenient  to  call  the  pressure  which  is  less 
than  atmospheric  negative,  and  that  which  is  greater  positive 
pressure. 

The  intrapulmonary  pressure  is  negative  during  inspiration 
and  positive  during  expiration.  Now,  owing  to  conditions 
already  referred  to,  as  the  chest  and  lungs  expand  during 
inspiration,  the  pressure  between  the  adjacent  walls  of  the  two 
(intrathoracic)  becomes  less  and  less  and  reaches  a  minimum 
at  the  end  of  that  act.  Furthermore,  owing  to  the  continuous 


PULMONARY    CAPACITY  147 

"pull"  of  the  elastic  lungs  upon  the  chest  walls  the  intrathoracic 
pressure  remains  negative  even  at  the  end  of  expiration.  But  it 
can  be  made  to  become  positive  under  forced  action  of  the 
expiratory  muscles,  as  in  coughing,  blowing,  etc.  The  constantly 
increasing  negative  condition  of  intrathoracic  pressure  is  evi- 
denced by  a  drawing  in  of  the  intercostal  tissues  during  inspira- 
tion; when  the  pressure  assumes  a  positive  character,  as  in  the 
expiratory  acts  of  the  pulmonary  emphysema,  these  tissues 
bulge  outward. 

Pulmonary  Capacity. — It  is  evident  that  the  most  forcible 
expiration  cannot  completely  empty  the  lungs  of  air.  The  air 
remaining  after  such  an  effort  is  the  residual  air.  It  amounts 
to  about  100  cubic  inches.  But  in  ordinary  respiration  at  the 
end  .of  the  expiratory  act  there  is  more  than  100  cubic  inches 
of  air  in  the  lungs,  because  in  such  cases  all  the  air  possible  is 
not  forced  out.  In  fact  about  200  cubic  inches  usually  remain; 
this  consists  of  the  residual  plus  another  100  cubic  inches,  which 
is  called  the  reserve  or  supplemental  air.  It  can  be  forced  out, 
but  is  not  in  tranquil  respiration.  The  amount  of  air  which  is 
taken  into  the  lungs  by  an  ordinary  respiratory  act  amounts  to 
about  20  cubic  inches,  and  is  termed  tidal  air.  It  is  the  only 
volume  used  in  quiet  breathing.  At  the  end  of  the  inspiratory 
act  in  tranquil  respiration  it  is  obvious  that  the  expansion  may 
continue  still  farther,  and  a  certain  amount  of  air,  over  and  above 
the  tidal  air,  be  taken  into  the  lungs.  The  maximum  amount 
which  can  be  so  inspired  (beyond  the  tidal)  is  about  no  cubic 
inches,  and  is  the  complemental  air. 

It  is  seen,  then,  that  the  entire  lung  capacity  is  equal  to  about 
330  cubic  inches.  But  the  residual  air  cannot  under  any  cir- 
cumstances be  called  into  use,  and  consequently  the  vital  capac- 
ity is  equal  to  the  total  capacity  minus  the  residual  air  (100  cubic 
inches),  or  230  cubic  inches.  It  is  the  volume  which  can  be  ex- 
pelled by  the  most  forcible  expiration  after  the  most  forcible 
inspiration. 


148  RESPIRATION 

The  capacity  of  the  trachea  and  larger  bronchi  is  known  as 
the  bronchial  capacity,  and  amounts  to  about  8  cubic  inches. 

The  quantity  of  air  in  the  small  bronchioles  and  air  vesicles 
is  increased  by  inspiration  and  decreased  by  expiration;  it  is 
called  alveolar  capacity,  and  at  the  end  of  ordinary  expiration 
amounts  to  about  150  cubic  inches.  Quiet  inspiration  increases 
it  to  about  1 80  cubic  inches. 

All  these  estimates,  of  course,  represent  only  an  average. 
The  vital  capacity  is  increased  by  stature,  by  any  occupation 
which  calls  for  active  physical  work  and  by  various  other 
conditions. 

Composition  of  Air. — Ordinary  atmospheric  air  contains,  in 
round  numbers,  about  21  parts  of  oxygen  to  79  parts  of  nitrogen. 
These  two  gases  make  up  the  main  bulk  of  the  atmosphere.  In 
addition,  the  atmosphere  always  contains  a  little  carbon  dioxide 
(about  .04  per  cent.),  ammonia,  moisture,  organic  material, 
dust,  nitric  acid,  etc.  All  except  the  oxygen  and  nitrogen  are  of 
minor  importance  in  respiration  when  they  are  not  present  in 
amounts  beyond  the  usual.  It  will  be  seen  that  the  striking 
difference  between  inspired  and  expired  air  is  in  the  proportions 
of  oxygen  and  carbon  dioxide. 

Diffusion  in  the  Lungs. — The  expired  air  contains  much 
more  CO2  and  much  less  O  than  the  inspired  air.  The  inter- 
change of  gases  between  the  alveolar  air  and  the  blood  is  re- 
sponsible for  the  difference. 

The  question  is  what  forces  cause  the  O  of  the  air  to  enter  the 
alveoli  and  the  CO2  to  leave  it.  As  might  be  supposed,  the  air 
escaping  during  the  first  part  of  expiration  differs  very  little  in 
composition  from  the  inspired  air,  for  it  has  been  occupying  the 
upper  air  passages  where  no  interchange  occurs.  The  bronchial 
capacity  is  only  about  one-third  large  enough  to  accommodate  the 
tidal  air,  and  consequently  the  greater  part  of  it  must  come  from 
lower  down  in  the  lung  structure,  and  the  CO2  in  the  expired  air 
continuously  increases  until  the  end  of  the  act.  At  each  inspira- 


DIFFUSION  IN  THE    LUNGS  149 

tion  at  least  two-thirds  of  the  tidal  air  must  pass  into  the  small 
bronchi,  or  lower.  Thus  it  is  that  inspiration  and  expiration 
themselves,  taking  into  and  bringing  out  of  the  vesicles  (or  at 
least  the  bronchioles)  air  fresh  with  O  and  air  vitiated  with 
CO 3,  aid  very  materially  in  keeping  constant  the  composition  of 
the  alveolar  air. 

In  the  second  place,  the  cardia'c  movements  have  a  similar  effect, 
each  systole  decreasing  the  size  of  the  heart  and  inducing  a  fresh 
atmospheric  current  toward  the  deep  alveoli,  and  each  diastole 
forcing  a  like  current  of  vitiated  air  toward  the  trachea.  This 
force  is  not  inconsequential. 

In  the  third  place,  the  diffusibility  of  gases  under  known  phys- 
ical laws,  without  the  aid  of  any  such  movements  as  have  been 
described,  is  an  occurrence  in  connection  with  the  phenomenon 
in  question.  Every  gas,  under  ordinary  atmospheric  conditions, 
exerts  a  certain  pressure.  In  every  mechanical  mixture  of  gases 
(such  as  the  atmosphere)  each  individual  gas  exerts  a  part  of 
the  total  pressure — a  part  proportional  to  its  percentage  in  that 
mixture.  This  has  been  called  the  "partial  pressure"  of  that 
gas.  Since  O  is  present  in  ordinary  atmosphere  to  the  extent  of 
21  parts  per  hundred,  the  partial  pressure  of  oxygen  in  the  at- 
mosphere is  i^  of  the  total  pressure. 

Now,  in  the  air  of  the  alveoli  O  is  present  to  a  less  extent  than 
21  parts  per  hundred,  and  consequently  its  partial  pressure  in 
that  situation  is  less  than  in  the  trachea  and  bronchi.  The  re- 
sult is  that  O  continually  makes  its  way  from  the  point  of  higher 
pressure  (trachea  and  bronchi)  toward  the  point  of  lower  pres- 
sure (alveoli).  The  tendency  is  thus  to  establish  a  uniform 
partial  pressure  throughout  the  whole  respiratory  tract;  but 
this  is  never  done  during  life  because  the  partial  pressure  above 
is  being  continually  increased  by  the  introduction  of  new  O, 
and  below  is  being  continually  diminished  by  the  removal  of 
that  gas  from  the  alveoli  by  the  blood. 

In  case  of  CO2  opposite  conditions  prevail.     This  gas  is  being 


1 50  RESPIRATION 

continually  introduced  into  the  alveolar  air  from  the  blood,  and 
consequently  it  is  present  there  in  much  larger  quantities  than 
in  the  trachea  and  bronchi,  which  contain  newly  inspired  air. 
The  partial  pressure,  therefore,  of  CO2  in  the  alveoli  is  much 
higher  than  in  the  upper  respiratory  passages,  and  a  continual 
curreht  of  it  diffuses  upward  to  equalize  the  pressure;  this  is 
never  accomplished,  however,  for  reasons  of  similar  nature  to 
those  keeping  up  the  constantly  unequal  pressure  of  O. 

These  three  factors — respiratory  and  cardiac  movements  and 
the  natural  diffusion  of  gases — are,  therefore,  in  continual  opera- 
tion to  get  O  to  and  CO2  away  from  the  alveoli.  Under  their 
influence  the  composition  of  the  alveolar  air  remains  fairly 
uniform. 

Alterations  of  Air  in  the  Lungs. — These  are  chiefly:  (a) 
Loss  of  oxygen,  (b)  gain  of  carbon  dioxide,  (c)  elevation  of  tem- 
perature, (d)  gain  of  water,  (e)  gain  of  ammonia,  (/)  gain  of 
organic  matter,  (g)  gain  of  nitrogen,  (h)  loss  of  (actual)  volume. 
The  capital  changes  are  loss  of  O  and  gain  of  CO2. 

(a)  Loss  of  Oxygen. — The  air  in  passing  through  the  lungs 
loses   of   O  nearly  5  per  cent,  of  its  total  volume.     That  is, 
whereas  on  entering  it  contains  21  parts,  on  leaving  it  contains 
only  about  16  parts  per  hundred  of  this  gas.     Nearly  25  per  cent, 
of  the  total  volume  of  O  inspired,  therefore,  is  lost  in  the  lungs. 

When  the  respirations  are  18  to  the  minute,  and  20  cu.  in.  of 
air  are  inspired  at  each  breath,  the  amount  inspired  in  an  hour 
will  be  21,600  cu.  in.  Since  a  little  more  than  one-fifth  of  this 
air  is  O,  and  since  only  one-fourth  of  the  inspired  O  is  consumed, 
the  total  amount  necessary  for  an  hour  will  be  about  1,100  cu.  in. 
This  allows,  however,  for  no  muscular,  digestive  or  other  ac- 
tivity, and  the  amount  actually  necessary  is  larger  than  this. 

The  circumstances  which  call  for  an  increase  in  O  almost  in- 
variably cause  an  increase  in  the  output  of  CO2. 

(b)  Gain  of  Carbon  Dioxide. — The  amount  of  CO2  in  inspired 
air  is  about  .04  part  per  hundred  (4/100  per  cent.);  the  amount 


ALTERATIONS    OF  AIR   IN   THE    LUNGS 


in  expired  air  is  something  more  than  4  parts  per  hundred.  In 
round  numbers  then,  the  air  in  passing  through  the  lungs  gains 
of  CO 2  4  per  cent,  of  its  entire  volume.  This  is  in  periods  of 
rest  from  exercise,  digestion,  etc.  The  total  amount  discharged 
in  one  hour  is,  on  an  average,  about  1,000  cu.  in.  This  estimate 
should  probably  be  raised  to  1,200  cu.  in.  for  ordinary  activity, 
and  varies  according  to  many  conditions,  some  of  which  are 
rapidity  and  depth  of  respiration,  age,  sex,  digestion,  diet,  sleep, 
exercise,  moisture,  temperature,  season,  integrity  of  the  nerve 
supply,  etc. 

The  subjoined  table  from  Kirkes'  Physiology  compares  the 
composition  of  inspired  and  expired  air. 


Inspired  Air. 

Expired  Air. 

Oxygen  

20.96  vols.  per  cent. 

16.03  v°ls-  Per  cent. 

Nitrogen  
Carbonic  acid  
Watery  vapor  
Temperature  

79        vols.  per  cent. 
0.04  vols.  per  cent, 
variable, 
variable. 

79       vols.  per  cent. 
4.4    vols.  per  cent, 
saturated, 
that  of  body  (36°  C.). 

Conditions  Influencing  Output  of  CO2. — When  the  rapidity  of 
respiration  is  increasing,  the  depth  remaining  constant,  the 
percentage  of  CO2  in  the  expired  air  is  reduced  because  more  air 
is  respired,  but  the  total  quantity  in  any  given  time  is  increased. 
The  same  result  follows  an  increased  depth  and  a  constant  rate. 
With  a  diminished  rapidity  and  increased  depth  more  CO2  is 
exhaled  than  under  opposite  conditions. 

The  amount  of  CO2  exhaled  is  small  in  very  young  infants. 
But  soon  the  output  begins  to  increase,  and  in  males  continues 
to  do  so  up  to  about  thirty  years;  there  is  then  a  slight  decrease  up 
to  sixty,  and  afterward  a  considerable  decrease  to  death. 

In  the  female  the  output  is  less  than  in  the  male.     In  the  former 


152  RESPIRATION 

sex  the  increase  is  said  to  cease  at  puberty  and  to  remain  con- 
stant until  the  menopause,  after  which  time  it  increases  to  sixty 
and  diminishes  subsequently. 

During  digestion  the  quantity  is  considerably  increased.  This 
is  probably  due  to  the  muscular  activity  of  the  alimentary  tract, 
to  glandular  metabolism  and  to  changes  taking  place  in  the  food 
products. 

As  to  diet,  it  may  be  said  in  general  that  the  exhaled  CO2  is 
increased  in  quantity  by  the  taking  of  nitrogenized  foods,  tea 
and  coffee. 

The  influence  of  sleep  is  to  diminish  the  output. 

Muscular  exercise  is  very  efficient  in  increasing  the  amount  of 
CO2  exhaled;  in  fact,  this  explains  partly  the  variations  in  con- 
nection with  sex,  digestion,  sleep,  etc. 

A  high  degree  of  moisture  increases  the  exhalation,  as  does 
a  rise  in  body  temperature.  A  rise  in  external  temperature,  how- 
ever, has  an  opposite  effect. 

The  output  is  increased  in  spring  and  decreased  in  autumn. 

When  the  efferent  nerve  supplying  a  part  is  severed  the  pro- 
duction of  CO2  in  that  part  is  at  once  diminished. 

The  consumption  of  O  and  the  exhalation  CO2  bear  a  fairly 
constant  relation  to  each  other — any  condition  increasing  one  in- 
creasing the  other,  and  vice  versa.  The  facts,  therefore, which 
have  been  mentioned  as  governing  the  exhalation  of  CO2  may 
be  applied  to  the  consumption  of  O. 

(c)  Gain  in  Temperature. — When  the  body  temperature  is 
normal  and  the  external  atmospheric  temperature  aboutyo0  F., 
it  is  found  that  air  inspired  through  the  nose  and  expired  through 
the  mouth  has  its  temperature  raised  from  70°  to  about  95°; 
the  rise  is  less  when  inspiration  takes  place  through  the  mouth. 
The  last  air  of  expiration  is  warmer  than  the  first.  This  gain 
of  heat  while  the  air  is  in  the  lungs  needs  no  explanation  when 
it  is  remembered  that  the  average  temperature  of  the  tissues 
with  which  it  is  in  contact  is  98.5  F.,  or  higher. 


OXYGEN   CONSUMED  AND    CARBON   DIOXIDE    EXHALED      153 

(d)  Gain  of  Water. — This  water  is  in  the  form  of  vapor.     It  is 
natural  that  the  air  should  absorb  water  from  the  moist  surfaces 
with  which  it  is  in  contact.     The  capillary  network  with  which 
it  is  in  close  relation  supplies  moisture  to  the  mucous  membrane 
not  only  of  the  alveoli  but  of  the  entire  respiratory  tract.     One 
or  two  pounds  of  water  are  eliminated  thus  daily. 

(e)  Gain  of  Ammonia. — Ammonia  is  exhaled  in  small  quan- 
tity by  the  lungs.     It  is  insignificant  except  in  cases  of  suppressed 
kidney  action. 

(/)  Gain  of  Organic  Matter. — The  quantity  of  organic  matter 
exhaled  by  the  lungs  is  inconsequential  (unless  ventilation  be 
bad),  but  such  exhalation  does  occur  to  a  small  extent.  It  gives 
the  odor  to  the  breath. 

(g)  Gain  of  Nitrogen. — The  exhalation  of  this  gas  by  the 
lungs  is  of  no  respiratory  importance.  The  amount  is  said  to 
be  ito— sV  *ne  amount  of  oxygen  consumed.  An  occasional 
loss  of  nitrogen  has  been  observed. 

(h)  Decrease  of  (Actual}  Volume.— When  the  external  tem- 
perature is  below  about  90°  F.  the  volume  of  expired  air  is  a 
little  greater  than  that  of  the  inspired  air,  because  of  the  increase 
of  temperature  it  undergoes  in  passing  through  the  lungs.  But 
the  actual  volume  of  the  expired  air,  when  reduced  to  the  same 
temperature  as  the  inspired,  is  found  to  be  always  a  little  less 
than  that  of  the  latter.  It  is  estimated  that  from  ^Wo  °f  tne 
total  volume  of  the  inspired  air  is  thus  lost  in  respiration. 

Besides  the  substances  mentioned  as  being  exhaled  from  the 
lungs,  it  is  well  known  that  odorous  emanations  proceed  from 
them  after  garlic,  onions,  turpentine,  alcohol,  certain  drugs,  etc., 
have  been  taken  into  the  stomach. 

Relation  Between  Oxygen  Consumed  and  Carbon  Dioxide 
Exhaled. — A  given  volume  of  O  will  combine  with  carbon  to 
form  the  same  volume  of  CO2;  or  the  amount  of  O  in  a  given 
volume  of  CO2  is  equivalent  to  that  volume  when  set  free  from 
the  carbon.  A  cubic  foot  of  0  will  unite  with  carbon  to  form 


154  RESPIRATION 

a  cubic  foot  of  CO2;  or  a  cubic  foot  of  CO2  will  yield,  on  dis- 
sociation, a  cubic  foot  of  O. 

This  being  the  case,  if  all  the  O  consumed  in  the  lungs  were 
exhaled  therefrom  in  the  form  of  CO2,  the  amount  of  CO2 
exhaled  would  just  equal  the  amount  of  O  consumed.  But  the 
amount  of  consumed  O  is  about  5  per  cent,  of  the  inspired  air, 
while  the  amount  of  exhaled  CO2  is  only  about  4  per  cent,  of 
the  expired  air.  It  follows,  therefore,  that  i  per  cent,  of  the  vol- 
ume of  inspired  air  is  not  represented  by  the  CO2  exhaled  from 
the  lungs  and  skin.  The  relation  between  the  consumed  O 
and  the  exhaled  CO2  is  usually  expressed  as  the  "  respiratory 
quotient" — the  division  of  the  latter  by  the  former  giving  the 
quotient.  This  quotient  is  made  to  vary  by  many  circum- 
stances, though  for  any  considerable  period  its  average  is  about 
the  same. 

While  it  has  been  stated  that  the  O  absorbed  and  the  CO2 
produced  vary  together  usually,  they  are  in  a  certain  measure 
independent  of  each  other.  For  CO2  does  not  result  from  the 
immediate  union  of  O  with  carbon  of  the  carbohydrates  and  fats, 
but  may  be  stored  in  the  shape  of  complex  compounds,  which 
may  later  split  up  with  the  formation  of  CO2,  either  by  oxida- 
tion or  by  intramolecular  cleavage.  Furthermore,  more  O  is 
necessary  to  oxidize  (that  is,  to  form  carbon  dioxide)  some  mole- 
cules than  others.  A  fat  requires  considerably  more  O  to  pro- 
duce CO2  than  does  a  carbohydrate;  so  that  the  kind  of  food  in 
store  would  also  affect  the  respiratory  quotient. 

With  respect  to  the  O  which,  in  the  long  run,  is  not  represented 
in  the  CO2  exhaled  from  the  lungs  and  skin,  it  is  certain  that 
when  various  of  the  food  stuffs  are  broken  down  at  least  a 
part  of  it  is  appropriated  by  hydrogen  to  form  water. 

Source  of  Exhaled  Carbon  Dioxide. — The  increase  of  CO2 
in  expired  air  over  the  small  amount  contained  in  inspired  air  is 
derived  from  the  venous  blood  circulating  through  the  lungs.  It 
exists  in  that  blood  under  a  constant  tension,  as  is  demonstrated 


CONDITION   OF    CO2   IN   THE    BLOOD  155 

by  its  escape  when  the  blood  is  placed  in  a  vacuum.  The  total 
amount  escapes  when  the  blood  intact  is  placed  in  vacuo:  when 
the  corpuscles  alone  are  so  treated  they  yield  up  all  their  CO2, 
though  it  is  small  in  amount ;  but  the  plasma  alone  in  vocuo  yields 
a  less  amount  than  when  it  contains  corpuscles.  If,  now,  corpus- 
cles be  added  to  the  plasma  the  total  amount  of  CO2  is  forth- 
coming. The  corpuscles  must,  therefore,  act  as  an  acid  causing 
the  liberation  of  this  gas  from  the  plasma.  It  is  probably  the 
hemoglobin,  or  oxyhemoglobin,  which  has  this  effect,  though  in 
the  laboratory  the  phosphates  and  certain  proteids  of  the  corpus- 
cles produce  a  like  reaction  when  brought  in  contact  with  the 
carbonates  and  bicarbonates  of  soda. 

Condition  of  CO2  in  the  Blood. — About  5  per  cent,  of  the 
total  amount  of  CO2  in  venous  blood  is  in  simple  solution  in  the 
plasma;  about  75-85  per  cent,  is  in  loose  chemical  combination 
in  both  corpuscles  and  plasma;  the  remaining  10-20  per  cent, 
is  in  comparatively  stable  combination  in  the  plasma.  Of  the 
75-85  per  cent.,  by  far  the  largest  part  is  in  the  plasma,  prob- 
ably in  a  condition  of  loose  association  with  sodium  to  form 
carbonates  and  bicarbonates;  the  small  part  in  the  corpuscles 
may  exist  in  a  similar  state,  but  it  is  now  thought  to  exist  in  com- 
bination with  the  proteid  portion  of  hemoglobin.  The  total 
75-85  per  cent,  in  corpuscles  and  plasma  is  so  loosely  combined 
that  the  mere  diminution  in  pressure  in  the  lungs  is  probably 
sufficient  to  liberate  it.  The  10-20  per  cent,  in  firm  chemical 
combination  is  that  part  which  cannot  be  extracted  from  plasma 
alone  in  vacuo,  but  wh'ich  is  dissociated  on  the  addition  of  an 
acid,  or  corpuscles,  or  hemoglobin,  etc.  It  may  be  that  as  the 
blood  passes  through  the  lungs  there  is  set  free,  in  the  formation 
of  oxyhemoglobin,  an  acid  which  immediately  unites  with  the 
bases  holding  the  CO2  in  combination — the  liberation  of  the 
latter  being  the  consequence. 

The  O  being  thus  in  the  air  vesicles,  and  the  CO2  thus  free,  or 
set  free,  in  the  blood,  with  the  very  thin  animal  membrane  con- 


156  RESPIRATION 

sisting  of  the  vesicular  and  capillary  walls  between  them,  it 
remains  to  be  seen  what  forces  are  concerned  in  the  interchange 
of  these  gases.  It  has  been  noted  that  only  one-fourth  of  the  O 
entering  the  lungs  in  the  air  is  taken  up  by  the  blood;  so  it  is  to 
be  remembered  that  not  all  the  CO2  entering  the  lungs  in  the 
venous  blood  is  taken  up  by  the  air. 

Interchange  of  Oxygen  and  Carbon  Dioxide  in  the  Lungs. 
— The  condition  of  "partial  pressure"  of  gases  in  mixture  has 
been  mentioned.  Each  gas  exerts  a  pressure  in  proportion  to 
its  percentage  in  the  mixture,  and  this  is  called  its  "partial  pres- 
sure." Now,  the  extraction  of  O  and  CO2  from  the  blood  by 
placing  it  in  a  vacuum  shows  that  both  these  gases  exist  in  the 
blood  under  a  certain  degree  of  tension. 

The  tension  of  a  gas  in  solution  being  only  the  pressure  nec- 
essary to  keep  it  in  solution,  it  follows  that  if  the  pressure  be 
diminished  the  gas  will  partly  escape.  If  an  atmosphere  con- 
taining, say,  O  at  a  certain  partial  pressure  be  brought  in  con- 
tact with  a  fluid  containing  O  at  a  certain  tension,  unless  the 
partial  pressure  of  the  O  in  the  air  be  equal  to  its  tension  in  the 
fluid  there  will  be  an  escape  of  the  gas  from  the  point  of  higher 
to  the  point  of  lower  pressure  or  tension.  If  the  partial  pressure 
of  the  gas  be  less  in  the  atmosphere  than  its  tension  in  the  fluid, 
the  current  will  be  from  the  latter  to  the  former  and  vice  versa. 
This  will  be  the  case  whether  the  media  are  in  actual  contact  or 
separated  by  an  animal  membrane. 

This  is  the  condition  which  obtains  in  the  pulmonary  alveoli. 
The  partial  pressure  of  O  in  the  alveolar  air  is  much  greater 
than  the  tension  of  O  in  the  blood;  consequently  the  current  is 
from  the  air  to  the  blood.  The  tension  of  CO2  in  the  venous 
blood  is  much  greater  than  the  partial  pressure  of  the  CO2  in  the 
alveolar  air;  consequently  the  current  is  from  the  blood  to  the  air. 

But,  here,  as  in  the  last  analysis  of  almost  all  physiological 
phenomena,  it  is  found  that,  while  these  purely  physical  laws 
are  certainly  concerned  in  the  pulmonary  interchange  of  gases, 


CONDITION    OF    OXYGEN    IN    THE    BLOOD  157 

they  are  insufficient  to  explain  the  occurrence  in  full.  For  the 
blood  will  take  from  the  alveolar  air  more  than  enough  O  to 
establish  an  equilibrium  of  tension  and  partial  pressure;  the  ten- 
sion of  O  in  arterial  blood  is  higher  than  its  partial  pressure  in 
alveolar  air.  So  it  is  found  that  the  alveolar  air  will  remove  more 
than  enough  CO2  to  establish  a  similar  equilibrium  of  this  gas. 
It  is  known  that  the  avidity  (chemical)  of  corpuscles  for  O  to 
form  oxyhemoglobin  causes  the  blood  to  appropriate  more  O 
than  it  would  otherwise  do,  but  even  then  we  are  driven  to  the 
usual  ultimatum  of  ascribing  some  peculiar  office  to  the  living 
epithelium  of  the  intervening  membrane. 

Condition  of  Oxygen  in  the  Blood.— Almost  all  the  oxygen 
is  conveyed  in  the  blood  by  the  red  corpuscles,  where  it  exists  in 
rather  unstable  composition  with  hemoglobin  (probably  with 
its  pigment  portion)  under  the  name  of  oxyhemoglobin.  Only  a 
comparatively  small  part  is  held  in  solution  by  the  plasma.  Dis- 
sociation of  oxyhemoglobin  occurs  when  the  pressure  is  suffi- 
ciently reduced. 

Alterations  in  Blood  in  Passing  Through  the  Lungs. — 
The  sum  total  of  the  changes  taking  place  in  the  blood  as  it  passes 
through  the  lungs  is  represented  by  the  term  arterialization.  In 
general,  it  may  be  said  that  the  blood  undergoes  changes  exactly 
opposite  to  those  of  the  air  in  circulating  through  the  pulmonary 
structure,  and  reference  to  the  list  of  substances  gained  and  lost 
by  the  air  will  suggest  the  main  alterations  in  the  blood. 

Of  course  the  most  striking  phenomena  are  the  loss  of  CO2 
and  the  gain  of  O.  In  100  volumes  of  arterial  or  venous  blood 
there  are  found  to  be,  on  an  average,  60  volumes  of  O  and  CO2. 
This  total  remains  approximately  constant,  though  the  relative 
amount  of  each  gas  varies  according  as  the  blood  is  venous  or 
arterial,  and  in  venous  blood  under  the  influence  of  several  con- 
ditions to  be  mentioned.  In  arterial  blood  the  O  will  represent 
about  20,  and  the  CO2  about  40,  of  the  total  60  volumes  per 
hundred  of  gas.  In  ordinary  venous  blood  the  O  will  represent 


158  RESPIRATION 

about  7  volumes  less  (13)  and  the  CO2  about  7  volumes  more  (47) 
of  the  total  60.  In  both  venous  and  arterial  blood  there  is  an 
insignificant  amount  of  nitrogen,  which  is  usually  present  to  the 
extent  of  1.5  volumes  per  hundred. 

The  proportion  of  gases  is  about  the  same  in  arterial  blood 
taken  from  any  part  of  the  system.  In  blood  coming  from  ac- 
tively secreting  glands  the  ratio  of  O  to  CO2  is  nearly  the  same 
as  in  arterial  blood;  in  fact,  such  blood  may  have  a  red  (arterial) 
instead  of  a  blue  (venous)  color.  This  is  because  during  activity 
blood  is  sent  to  the  gland  in  increased  amount  to  furnish  materials 
for  secretion,  while  the  demand  for  oxygen  is  not  relatively 
increased  in  that  gland. 

Besides  the  changes  which  are  apparent  on  referring  to  the 
alterations  in  the  air  passing  through  the  lungs,  there  are  certain 
other  general  characteristics  which  distinguish  arterial  from 
venous  blood.  The  most  noticeable  is  color.  Venous  blood  is 
changed  in  the  lesser  circulation  from  a  dark  blue,  or  black,  to  a 
bright  red.  This  is  due  to  the  formation  of  oxyhemoglobin.  The 
change  of  color  does  not  occur  when  the  appropriation  of  O  is 
interfered  with,  as  when  the  air  is  excluded  from  the  lungs,  or 
when  carbon  monoxide  is  inhaled.  Again,  there  is  every  reason  to 
believe  that  venous  blood  coming  from  different  organs  differs  in 
composition  according  to  the  special  materials  which  have  been 
extracted  from  it  by  those  organs;  the  portal  blood  during  diges- 
tion must  certainly  be  different  in  composition  from  the  general 
venous  blood,  and  so  it  may  be  conceived  that  the  blood  coming 
from  no  two  different  sets  of  capillaries  is  identical.  When  all 
this  meets  in  the  right  side  of  the  heart  and  is  sent  thence  into 
the  lungs  it  has  a  nearly  uniform  composition,  and  needs  only 
to  receive  O  before  it  can  supply  the  wants  of  any  particular 
tissue  in  the  body.  Arterial  blood  is  also  more  coagulaUe  than 
venous. 

Internal  Respiration. — It  has  been  said  that  the  object  of 
external  respiration  and  the  transportation  of  O  and  CO2  is  to 


OXYGEN  AND    CARBON   DIOXIDE   IN   THE    TISSUES  159 

make  internal  respiration  possible.  Oxygen,  leaving  the  alveoli 
in  a  manner  already  described,  enters  the  blood  and  at  once  com- 
bines with  hemoglobin  of  the  red  corpuscles  to  form  oxyhemo- 
globin.  A  small  portion  of  the  O  is  used  up  by  the  corpuscles  in 
transit,  with  the  production  of  CO2  and  other  metabolic  mate- 
rials— the  corpuscles  requiring  O  in  their  metabolism  just  as  do 
other  cells.  But  by  far  the  largest  portion  is  carried  to  the 
capillaries,  where  it  is  taken  up  by  the  cells.  At  the  same  time 
the  cells  give  up  to  the  blood  CO., — a  result  of  their  metabolic 
activity.  The  blood,  having  thus  given  up  its  O,  is  changed  in 
color,  and  carries  the  CO?  back  to  the  lungs  to  be  exhaled. 

To  furnish  O  and  to  remove  CO2  is  the  only  object  of  respira- 
tion. Living  tissue  exposed  to  an  atmosphere  containing  O  will 
consume  O  and  exhale  CO2  even  if  no  blood  be  circulating  through 
it.  The  exact  manner  in  which  a  cell  uses  O  is  not  apparent. 
It  is  evidently  an  oxidation  process  which  produces  CO2,  and  O 
is  directly  necessary  to  this  process.  But  the  amount  of  CO2 
produced  in  any  given  time  may  not  correspond  to  the  amount  of 
O  consumed  in  that  time ;  it  may  be  greater  or  less.  "  It  is  prob- 
able that  during  rest  O  is  utilized  to  some  extent  in  oxidations 
which  are  not  at  once  carried  to  their  final  stage  and  in  which 
relatively  little  CO2  is  formed;  hence  during  activity  compara- 
tively little  O  is  required  to  cause  a  final  disintegration  of  the 
now  partially  broken  down  substances,  and  thus  to  give  rise  to 
a  relatively  large  formation  of  CO2"  (Reichert). 

The  absorption  of  O  is  to  be  looked  upon  as  a  part  of  the 
nutritive  process  just  as  the  absorption  of  proteid,  e.  g.,  and  CO2 
as  one  of  the  products  of  destructive  metabolism  just  as  urea. 
There  is  small  probability  that  the  O  unites  directly  with  the 
carbon  of  any  of  the  food  stuffs — although  this  is  the  final 
result. 

Interchange  of  Oxygen  and  Carbon  Dioxide  in  the  Tis- 
sues.— Here  application  of  the  principles  governing  the  inter- 
change of  these  gases  in  the  lungs  applies.  It  is  found  that 


160  RESPIRATION 

the  tissues  act  as  very  strong  reducing  agents  upon  oxyhemo- 
globin,  setting  free  the  O.  Now  the  tension  of  O  in  the  arterial 
capillaries  is  much  higher  than  in  the  tissues;  in  fact,  it  is  prac- 
tically nothing  in  the  latter  situation,  for  the  O  enters  so  quickly 
into  combination  that  there  is  very  little  to  be  found  here  at  any 
time.  Consequently  physical  laws  encourage  the  passage  of 
this  gas  out  of  the  capillaries  into  the  tissue. 

On  the  other  hand,  the  tension  of  CO2  in  the  tissues  is  much 
higher  than  in  the  blood,  and  the  same  physical  laws  encourage  a 
current  of  CO2  toward  the  blood.  Nevertheless,  these  laws  do 
not  explain  all  the  phenomena  of  interchange;  the  activity  of 
the  cells  is  an  important  agent,  though  their  influence  may  be 
of  a  chemical  nature  only. 

Cutaneous  Respiration. — Cutaneous  respiration  in  man  is  in- 
significant and  not  essential  to  life.  The  skin  absorbs  a  little  O 
and  exhales  a  little  more  CO2.  It  is  estimated  by  Scharling  that 
the  skin  performs  about  -^  of  the  respiratory  function.  Death 
following  the  covering  of  the  body  surface  with  an  impermeable 
coating  is  not  due  to  interference  with  cutaneous  respiration. 

Ventilation. — -Persons  breathing  in  a  confined  space  gradually 
consume  the  O  and  increase  the  CO2  of  the  atmosphere.  When 
the  amount  of  O  has  been  decreased  to  fifteen  parts  per  hundred 
it  is  insufficient  for  the  respiratory  demands.  When  the  CO2  is 
increased  to  .07  part  per  hundred  the  air  becomes  disagreeable 
and  close;  this  is  not,  however,  from  the  accumulation  of  CO2  so 
much  as  from  organic  emanations  and  disagreeable  odors  from 
the  body,  clothing,  etc.  It  is  only  that  the  amount  of  CO2  serves 
as  an  indication  of  the  extent  of  accumulation  of  these  materials 
that  the  amount  of  .07  per  cent,  is  fixed  as  the  limit  beyond  which 
it  ought  not  to  be  present.  This  percentage  of  CO2  in  air  free 
from  emanations,  etc.,  is  not  deleterious. 

Since  1,200  cu.  in.  of  O  are  consumed  per  hour,  about  15  cu. 
ft.  will  be  necessary  for  a  day;  and  since  the  1,200  cu.  in.  con- 
sumed represent  only  about  one-fourth  of  the  O  inspired,  60 


ABNORMAL    RESPIRATION  l6l 

cu.  ft.  will  be  necessary  for  inspiration  during  twenty-four  hours. 
This  amount  represents  some  300  cu.  ft.  of  atmospheric  air — 
which  an  ordinary  person  must  have  in  that  time. 

But  this  estimate  allows  nothing  for  increased  respiratory  ac- 
tivity, which  inevitably  occurs  from  some  of  the  numerous  con- 
ditions influencing  it.  It  is  found  that  in  prisons  and  other  in- 
stitutions of  confinement  it  is  not  safe  to  allow  each  person  less 
than  1,000  cu.  ft.  of  atmospheric  air.  In  crowded  houses,  where 
this  space  per  individual  cannot  be  obtained,  it  is  necessary,  in 
order  to  avoid  unpleasant  results,  to  change  the  air  continuou'sly, 
or  at  frequent  intervals,  Natural  and  artificial  means  are  em- 
ployed to  accomplish  this  end. 

Respiration  of  Various  Gases. — The  inhalation  of  pure 
oxygen  is  not  deleterious  unless  it  be  under  higher  tension  than 
in  atmospheric  air,  when  it  becomes  a  local  irritant.  The  blood 
will  not,  however,  appropriate  more  than  the  usual  amount. 
Nitrous  oxide  will  sustain  respiration  for  a  time,  but  soon  pro- 
duces unconsciousness  and  asphyxia,  probably  because  it  unites 
so  firmly  with  the  hemoglobin  of  the  corpuscles.  Hydrogen  may 
be  inhaled  with  impunity  if  it  contain  also  oxygen  in  the  atmos- 
pheric proportion.  Carbon  monoxide  is  poisonous  because  it 
unites  with  hemoglobin  to  the  exclusion  of  oxygen  and  will  not 
dissociate  itself.  Sulphurreted,  phosphoretted  and  arseniuretted 
hydrogen  are  destructive  of  hemoglobin  and  consequently  poison- 
ous. Pure  carbon  dioxide  cannot  be  inhaled  for  any  length  of  time. 

Abnormal  Respiration. — The  term  eupnea  is  used  to  describe 
normal,  tranquil  breathing.  Apnea  is  suspended  respiration. 
Hyperpnea  is  exaggerated  respiration.  Dyspnea  is  labored 
breathing.  Asphyxia  is  essentially  a  want  of  O  characterized 
by  convulsive  respirations,  and  later  by  irregular  shallow  breath- 
ing. The  last  two  named  deserve  some  attention. 

Dyspnea  may  be  due  to  either  a  deficiency  of  O  or  an  excess 
of  CO2  in  the  blood.  When  an  animal  is  made  to  breath  in  a 
small,  confined  space  the  amount  of  O  soon  becomes  insufficient, 
ii 


162 


RESPIRATION 


even  though  the  amount  of  CO2  in  the  blood  be  not  increased. 
Again,  if  an  animal  be  caused  to  breathe  air  containing  the  usual 
amount  of  O  and  a  large  amount  of  CO2,  it  will  suffer  from  dysp- 
nea also.  In  either  case  the  manifestations  are  practically  the 
same — slow,  deep  and  labored  respiration.  In  cardiac  disease, 
hemorrhage,  pulmonary  diseases,  etc.,  the  dyspnea  is  from  a 
lack  of  O  in  the  tissues,  because  of  enfeebled  action  of  the  heart, 


FIG.  51. — The  heart  in  the  first  stage  of  asphyxia. 

The  left  cavities  are  seen  to  be  distended;  the  left  ventricle  partly  overlaps  the 
right;  La,  left  auricle;  l.v.  left  ventricle;  a,  aorta;  p.a.  pulmonary  artery;  p.v.  pul- 
monary vein;  r.a.  right  auricle;  r.v.  right  ventricle;  v.c.d.  descending  vena  cava; 
v.c.a.  ascending  vena  cava.  (Kirkes  after  Sir  George  Johnson.) 

deficient  quantity  of  blood,  insufficient  exposure  of  the  blood  in 
the  lungs,  etc. 

Asphyxia  may  be  looked  upon  as  exaggerated  dyspnea.  The 
labored  breathing  of  dyspnea  becomes  convulsive,  and  finally 
collapse  ensues.  Respiration  becomes  shallow,  consciousness 
is  lost,  the  pupils  are  dilated,  opisthotonus  develops,  the  re- 
flexes disappear,  and  at  last  the  heart  stops  beating.  The  skin 
and  mucous  membranes  become  blue  from  non-oxygenation  of 
the  blood.  Asphyxia  from  submersion  is  harder  to  overcome 
than  from  simple  deprivation  of  air  outside  the  water.  Resusci- 


RESPIRATION  AND   BLOOD-PRESSURE  163 

tation  is  extremely  doubtful  when  a  person  has  been  submerged 
as  long  as  five  minutes. 

While  the  phenomena  of  dyspnea  and  asphyxia  are  referable 
to  the  lungs,  it  is  not  the  need  of  air  in  these  organs,  but  of  O  in 
the  tissues,  which  gives  rise  to  the  symptoms.  The  non-oxygen- 
ated blood  in  asphyxia  will  not  circulate  through  the  capillaries 


FIG.  52. — The  heart  in  the  final  stage  of  asphyxia. 

The  letters  have  the  same  meaning  as  in  Fig.  51;  in  addition,  p.  c.  represents  the 
pulmonary  capillaries.  The  right  auricle  and  ventricle,  and  the  pulmonary  artery, 
are  fully  distended,  while  the  left  cavities  of  the  heart  and  the  aorta  are  nearly 
empty.  (Kirkes  after  Sir  George  Johnson.) 

except  with  the  greatest  difficulty,  and  the  result  is  that  it  accu- 
mulates in  the  arterial  system,  dams  back  upon  and  distends  the 
heart,  so  that  this  organ  is  finally  paralyzed  and  ceases  to  beat. 
This  is  the  cause  of  death  from  asphyxia. 

Effect  of  Respiration  on  Blood-Pressure. — The  lowest 
blood-pressure  is  just  after  the  beginning  of  inspiration,  from 
which  time  it  increases  during  inspiration  to  reach  its  maximum 
just  after  the  beginning  of  expiration;  it  gradually  decreases 
from  this  time  to  the  minimum  just  after  the  beginning  of  in- 
spiration. The  general  effect,  then,  of  inspiration  is  to  increase 


164 


RESPIRATION 


blood-pressure  and  of  expiration  to  decrease  it.     This  remark 
applies  to  general  arterial  tension. 

Taking  inspiration,  the  increase  in  arterial  tension  is,  in  its 
last  analysis,  due  to  the  larger  amount  of  blood  sent  into  the  arte- 
rial system  at  each  ventricular  systole.  The  explanation  is  some- 
what complex,  but  if  the  mechanics  of  respiration  be  under- 
stood it  may  be  made  satisfactory. 


FIG.  53. — Carotid  blood-pressure  tracing  of  a  dog. 
Vagi  not  divided;  I,  inspiration;  E,  expiration.     (Stirling.) 

It  was  seen  that  the  lungs  are  contained  in  an  air-tight  cavity, 
the  chest,  and  that  they  expand  with  the  chest  because  of  nega- 
tive pressure  (" suction")  exerted  upon  them.  The  heart  is  also 
a  hollow  organ  situated  in  this  cavity;  it  has  connected  with  it, 
and  lying  also  in  the  thoracic  cavity,  large  vessels  communicat- 
ing with  smaller  extrathoracic  vessels.  The  heart  and  these 
great  thoracic  vessels  are  elastic  and  distensible.  Consequently 
the  expansion  of  the  thorax  also  expands  them  slightly  and  tends 
to  draw  blood  from  the  extrathoracic  into  the  intrathoracic 
vessels  and  heart;  in  fact  inspiration  is  one  of  the  main  forces 
causing  a  flow  of  venous  blood  toward  the  heart.  Now  all  this, 
so  far  as  it  goes,  tends  to  keep  the  blood  out  of  the  extrathoracic 
vessels,  and  so  to  contradict  the  statement  that  inspiration 
increases  arterial  tension. 

But,  remembering  that  we  are  dealing  with  arterial  tension 


RESPIRATION  AND    BLOOD-PRESSURE  165 

and  that  our  effort  is  to  prove  that  more  blood  is  sent  into  the 
aorta  during  inspiration  than  during  expiration,  it  is  of  value  to 
note  that  since  the  walls  of  the  aorta  are  more  resistant  than 
those  of  the  venae  cavae  there  is  less  expansion  of  the  former  than 
of  the  latter  during  inspiration,  and  consequently  less  tendency  for 
the  arterial  blood  to  regurgitate  into  the  thoracic  aorta  than  for 
the  venous  blood  to  enter  the  thoracic  venae  cavae.  The  same 
expanding  force  dilates  the  pulmonary  capillaries,  pulmonary 
artery  and  pulmonary  veins — the  artery  least  of  these.  Taking 
it  for  granted  that  more  blood  is  being  received  by  the  right  side 
of  the  heart  from  the  incoming  venae  cavae,  the  somewhat  dilated 
pulmonary  artery  receives  more  from  the  right  ventricle;  the 
pulmonary  capillaries  are  more  dilated  than  the  artery  and  this 
fact  greatly  encourages  (by  a  suggestive  "suction")  the  increased 
flow  from  the  pulmonary  artery;  they,  therefore,  receive  more 
blood  than  usual.  The  pulmonary  veins,  being  likewise  dilated, 
exert  " suction"  upon  the  capillaries,  and  thus  receive  and  pass 
on  to  the  heart  a  larger  supply  of  blood  than  usual.  The  heart, 
receiving  more  blood,  must  send  more  into  the  aorta,  thereby 
increasing  arterial  tension  in  the  extrathoracic  vessels,  unless, 
by  expansion  of  the  chest,  the  thoracic,  aorta  be  so  dilated  as  to 
accommodate  the  increased  amount — which  is  not  true. 

Then,  finally,  the  validity  of  this  argument  will  hinge  on  the 
relative  dilatation  of  the  thoracic  aorta  and  of  the  thoracic  venae 
cavae.  If  the  veins  be  less  dilated  by  inspiration  than  the  artery, 
then  they  will  receive  an  increase  of  blood  which  will  not  com- 
pletely occupy  the  increase  of  space  in  the  dilated  thoracic  aorta, 
and  there  will  be  a  backward  " suction"  made  upon  the  contents 
of  the  arterial  tree  with  a  consequent  decrease  in  pressure;  but 
a  condition  just  opposite  to  this  seems  to  obtain. 

During  expiration  contrary  conditions  in  general  are  opera- 
tive with  contrary  results.  The  intrapulmonary  vessels  and 
heart  are  compressed,  but  the  veins  and  capillaries  more  than 
the  aorta,  with  the  result  that  less  blood  reaches  the  heart  than 


l66  RESPIRATION 

during  inspiration,  and  the  thoracic  aorta  being,  relatively  to  the 
thoracic  venae  cavae,  more  dilated  now  than  during  inspiration 
can  easily  accommodate  the  decreased  amount  of  blood  which  it 
receives.  Of  course  expiration  increases  venous  pressure  in  the 
veins  which  enter  the  thorax  back  as  far  as  the  valves. 

.  The  reason  the  pressure  does  not  rise  with  the  beginning  of 
inspiration  is  because  a  short  time  is  consumed  in  filling  the 
flaccid  intrapulmonary  veins,  and  the  first  increase  of  blood  is  de- 
layed for  that  purpose  instead  of  passing  on  to  the  left  side  of 
the  heart.  Similarly,  the  pressure  continues  to  rise  for  a  short 
time  after  expiration  has  begun  because  the  large  veins  are  being 
emptied  by  pressure  during  this  time  and  their  contents  are 
reaching  the  heart  and  being  forced  into  the  aorta. 

Movements  of  the  diaphragm  and  abdominal  muscles  during 
respiration  also  lend  themselves  to  create  like  changes  in  arterial 
pressure,  but  the  main  factors  are  intrathoracic. 

The  fact  that  the  cardiac  rate  is  increased  during  inspiration 
and  decreased  during  expiration  may  also  have  to  do  with  the 
variations  in  pressure. 

All  the  foregoing  remarks  relative  to  arterial  tension  are 
meant  to  apply  to  tranquil  respiration.  During  forced  inspira- 
tion, or  forced  expiration,  the  results  may  be  modified,  or  even 
reversed,  by  circumstances  not  necessary  to  mention. 

Nervous  Mechanism  of  Respiration. — Although  the  muscles 
of  respiration  are  of  the  striated  variety,  it  is  by  no  effort  of  the 
will  that  the  movements  are  kept  up.  They  belong  to  the  class 
known  as  automatic;  that  is,  they  are,  up  to  certain  limits,  under 
the  control  of  the  will,  but  recur  in  a  regular,  coordinate  and 
orderly  manner  without  the  active  intervention  of  volition. 
Respiration  represents  the  activity  of  a  self-governing  apparatus. 
These  movements  constitute  a  finely  coordinated  set  of  contrac- 
tions— contractions  which  are  regulated  by  means  of  afferent 
and  efferent  nerves  under  the  supervision  of  the  respiratory 
center. 


NERVOUS   MECHANISM    OF    RESPIRATION  167 

The  respiratory  center  is  in  the  lower  part  of  the  medulla 
oblongata.  Destruction  of  the  encephalon  above,  or  the  cord 
below,  the  center  does  not  arrest  respiration.  It  is  bilateral — a 
center  for  each  side—  and  these  are  more  or  less  independent  of 
each  other,  but  are  so  intimately  connected  by  commissural 
fibers  that  any  impression  made  upon  one  usually  produces  a 
like  effect  upon  the  other.  Each  half  presides  over  the  lungs 
and  respiratory  muscles  of  its  own  side,  but  acts  synchronously 
with  its  fellow  of  the  opposite  side.  Furthermore,  each  of  these 
lateral  centers  may  be  regarded  as  consisting  of  two  parts,  one 
for  inspiration  and  one  for  expiration.  Stimulation  of  the  in- 
spiratory  center  not  only  strengthens  the  inspiratory  act,  but 
also  accelerates  respiration.  Stimulation  of  the  expiratory 
center  strengthens  expiration  and  also  retards  the  respiratory 
rate.  The  accelerator  portion  of  the  center  seems  more  sensi- 
tive than  the  inhibitory,  and  the  result  of  stimulation  of  the 
whole  center  is  therefore  quickened  respiration. 

Subsidiary  respiratory  centers  are  said  to  exist  in  the  tuber 
cinereum,  optic  thalamus,  corpora  quadrigemina,  pons  Varolii 
and  spinal  cord;  but  the  existence  of  at  least  some  of  these  is 
doubtful. 

Rhythm  of  Respiration. — What  agency  excites  the  center  to 
keep  up  the  respiratory  movements  with  such  regularity  is  a 
matter  of  interest.  The  chief  circumstances  which  seem  to 
affect  the  rate  and  rhythm  are  (i)  the  will,  (2)  emotions,  (3)  com- 
position of  the  blood  and  (4)  afferent  impressions. 

i,  2.  The  effect  of  the  will  and  emotions  are  too  apparent 
.  to  call  for  comment,  i  and  2  are  properly  included  in  4. 

3.  A  deficiency  of  O  or  an  excess  of  CO2  in  the  blood  will 
increase  the  rate.     Increase  in  temperature  of  the  blood,  as  in 

ever,  will  produce  a  similar  effect. 

4.  The  most  important  of  these  agencies  is  found  in  afferent 
impressions  conveyed  to  the  center.     The  fibers  carrying  these 

mpressions  are  chiefly  in  the  pneumo  gastric,  glosso-pharyngeal, 


1 68  RESPIRATION 

trigeminal  and  cutaneous  nerves.     Of  these  the  pneumogastric 
is  by  far  the  most  important. 

Section  of  a  single  pneumogastric  is  followed  by  variable  res- 
piratory disturbances  which  usually  disappear  in  less  than  an 
hour.  Section  of  both  nerves  is  followed,  after  a  short  interval 
of  increased  respiratory  activity,  by  slow  and  powerful  inspira- 
tions, by  forced  expiration  and  an  appreciable  interval  before 
the  next  inspiration.  Irritation  of  the  central  end  of  the  cut 
nerve  by  a  very  weak  current  seems  to  stimulate  the  inhibitory 
part  of  the  center,  for  the  rate  is  slowed,  the  expirations  are 
strenuous  and  the  inspirations  weak.  When  the  current  is 
increased  to  a  moderate  strength  opposite  results  are  obtained, 
the  accelerator  portion  of  the  center  being  stimulated.  These 
facts  show  that  the  pneumogastrics  possess  both  inspiratory  and 
expiratory  fibers,  and  that  the  former  are  stimulated  more  by  a 
moderate  current  and  the  latter  more  by  a  very  weak  one. 
The  rhythm  of  respiration,  therefore,  includes  the  regular 
sequence  of  inspiratory  and  expiratory  movements  upon  each 
other. 

Now  what  is  it  that,  under  normal  conditions,  irritates  the 
terminals  of  the  pneumogastrics  and  causes  them  to  convey 
inspiratory  and  expiratory  impressions?  It  has  been  held  that 
a  change  in  the  composition  of  the  alveolar  air — an  accumulation 
of  carbon  dioxide — irritates  the  nerve  terminals  and  explains 
the  conveyance  of  the  inspiratory  impressions,  while  the  stretch- 
ing of  the  lung  tissue  originates  the  expiratory  impressions. 
Others  ascribe  both  inspiratory  and  expiratory  impressions  to 
lung  movements — movements  of  inspiration  exciting  expiratory, 
fibers,  and  movements  of  expiration  exciting  inspiratory  fibers. 
These  observers  cite  the  fact  that  artificial  inflation  and  aspira- 
tion excite  expiration  and  inspiration  respectively. 

Stimulation  of  the  superior  laryngeal,  as  when  foreign  bodies 
accidentally  enter  the  larynx,  excites  violent  expiration. 

The  glosso-pharyngeal  contains  afferent  fibers  especially  impor- 


NERVOUS   MECHANISM    OF    RESPIRATION  169 

tant  in  arresting  respiration — at  any  stage  whatever — during  the 
act  of  deglutition. 

Stimulation  of  the  sensory  fibers  of  the  trigeminal  in  the  nose, 
as  by  irritating  vapors,  may  arrest  respiration. 

Irritation  of  the  cutaneous  nerves  in  general,  as  by  cold  or  hot 
water,  slapping,  etc.,  stimulates  respiratory  movement. 

There  are,  of  course,  running  from  the  cortex  to  the  respira- 
tory center  intracranial  fibers  whereby  the  organ  of  the  will 
makes  its  presence  felt  in  respiration. 

But  when  all  the  afferent  nerve  connections  are  severed,  respi- 
ration continues  with  modified  rhythm  and  rate,  at  least  for  a  time. 
It  is  thought  that,  under  these  conditions,  it  is  the  circulation 
through  the  center  of  blood  deficient  in  oxygen  which  causes  the 
cells  to  discharge ;  that  is,  after  every  inspiration  and  subsequent 
expiration  there  is  not  another  inspiration  until  the  blood  has 
become  sufficiently  deoxygenated,  or  charged  with  carbon 
dioxide,  to  irritate  the  respiratory  center. 

We  may  conclude  that  "the  rhythmical  discharges  from  the 
center  are  due  primarily  to  an  inherent  quality  of  periodic  ac- 
tivity of  the  nerve  cells  constituting  the  respiratory  center,  and 
maintained  by  the  blood,  and  that  the  rhythm,  rate,  and  other 
characters  of  these  discharges  may  be  affected  by  the  will  and 
the  emotions,  by  the  composition,  supply  and  temperature  of 
the  blood,  and  by  various  afferent  impulses.  The  chief  factors 
are  the  quantities  of  O  and  CO2  in  the  blood,  and  the  impulses 
conveyed  from  the  lungs  by  the  fibers  of  the  pneumogastric 
nerves.'*  (Am.  Text-Book.) 

The  efferent  nerves  of  respiration  control  the  muscular  move- 
ments of  that  act.  They  are  chiefly  the  facial,  hypoglossal  and 
spinal  accessory  controlling  the  respiratory  movements  about  the 
face  and  throat;  the  pneumogastric  going  to  the  larynx;  the 
phrenic  to  the  diaphragm  certain  of  the  spinal  nerves. 

To  the  lungs  proper  fibers  are  distributed  by  the  vagus,  the 
dorsal  spinal  and  the  sympathetic  nerves.  Besides  the  expiratory 


1 70  RESPIRATION 

and  inspiratory  fibers  already  noticed,  the  vagus  supplies  the 
lungs  with  broncho-motor,  general  sensory,  trophic  and  secre- 
tory (mucous)  fibers.  The  sympathetic  furnishes  trophic  and 
vaso-motor  fibers,  which  latter  come  from  the  cord  by  the  roots 
of  the  dorsal  nerves  mentioned  to  join  the  sympathetic  ganglia. 


CHAPTER  IX. 

NUTRITION,  DIETETICS  AND  ANIMAL  HEAT. 
NUTRITION. 

ALL  the  processes  of  the  body  as  digestion,  absorption,  secre- 
tion, circulation,  respiration,  etc. — have  a  single  object,  viz.,  the 
nutrition  of  the  cells  of  the  body. 

The  ultimate  source  of  all  nutriment  is,  of  course,  food  and 
oxygen.  The  oxygen  has  been  followed  from  the  lungs  to  the 
tissues  as  oxyhemoglobin  of  the  blood.  The  various  foods  have 
been  seen  to  disappear  from  the  digestive  tract  and  to  be  con- 
veyed to  the  tissues  by  the  great  nutritive  fluid,  some  in  recog- 
nizable and  some  in  unrecognizable  form.  If,  now,  we  shall  be 
able  to  discover  in  what  way  these  different  materials  thus  fur- 
nished the  cells  are  utilized  and  appropriated  by  them,  and  in 
what  condition  they  subsequently  escape  from  the  system,  the 
study  of  nutrition  will  have  been  rendered  much  clearer.  The 
intake  is  through  the  lungs  and  alimentary  canal;  the  output  is 
mainly  by  the  lungs,  skin,  kidneys,  and  intestines.  To  show  for 
the  changes  which  take  place  while  the  food  is  in  the  body  there 
is  the  growth  of  the  body,  the  maintenance  of  tissue  integrity, 
secretion,  heat,  motion  and  nervous  energy. 

It  may  be  said  at  once,  however,  that  the  exact  method  of 
appropriation  of  nutritive  material  by  the  tissues  is  a  subject  of 
speculation,  since  it  involves  the  question  of  life  itself;  and  we 
shall  have  to  be  content  with  recounting  some  of  the  condi- 
tions influencing  and  some  of  the  phenomena  attendant  upon 
the  process. 

171 


172  NUTRITION,    DIETETICS  AND  ANIMAL  HEAT 

Metabolism. — By  metabolism  is  meant  those  processes  in  the 
body  whereby  food  products  are  appropriated,  their  stored-up 
energy  utilized,  and  the  waste  discarded. 

Metabolism  is  divided  into,  (i)  anabolism,  and  (2)  katabolism. 
Anabolism  is  the  process  of  building  up  tissue  by  cell  appro- 
priation of  food  stuffs.  Katabolism  is  the  process  of  destroying 
tissue  in  order  to  set  free  energy  that  the  organs  of  the  body 
may  perform  their  various  functions. 

When  the  anabolic  processes  are  equal  to  the  katabolic  there 
is  no  excessive  storage  of  material,  but  an  individual  remains 
of  uniform  size,  weight,  and  strength.  If  the  anabolic  are  in 
excess  of  the  katabolic  processes,  the  excessive  products  are 
stored  up  in  cells  and  an  individual  increases  in  size,  weight 
and  strength.  If  the  katabolic  processes  are  in  excess  of  the 
anabolic  there  is  a  call  on  the  tissues  for  the  matter  already 
stored  there  and  there  is  a  decrease  in  strength,  weight  and 
size. 

Death. — As  long  as  a  cell  appropriates  enough  to  supply  the 
deficit  caused  by  the  destruction  of  material  in  the  expenditure 
of  energy,  the  cell  will  live ;  but  when  the  intake  cannot  make  up 
for  the  output  lost  the  cell  ceases  to  functionate  and  this  is  called 
death. 

Problems  Involved  in  the  Nutritive  Process. — Since  the 
actual  changes  occurring  and  the  method  of  their  production 
cannot  be  understood,  the  question  of  nutrition  resolves  itself 
into  a  consideration  of  the  final  fate  of  the  various  aliments,  of 
their  relative  value  in  nutrition,  of  conditions  influencing  the 
process,  and  of  the  explanation  of  certain  facts  connected  with 
the  destruction  of  the  food-stuffs,  particularly  the  production 
of  heat. 

The  change  which  the  foods  finally  undergo  in  the  body  is  one 
of  oxidation.  It  is  therefore  chemical  changes  which  give  rise 
to  physical  activity.  Oxidation  is  accompanied  by  the  produc- 
tion of  heat.  The  same  sum  total  of  heat  is  developed  when  a 


FOODS   IN   NUTRITION  1 73 

piece  of  iron  rusts  completely  away  in  five  years  as  when  it  is  con- 
sumed in  an  atmosphere  of  oxygen  in  five  minutes.  In  both 
cases  it  is  oxidized  In  the  cell  oxidation  is  continually  going 
on  with  the  production  of  heat  and  of  certain  excrementitious 
(oxidation)  products  depending  on  the  kind  of  food-stuffs. 

Fate  of  Different  Foods  in  the  Organism. — In  the  first 
place,  the  foods  may  be  divided,  into  (I)  those  which  pass  through 
the  organism  unchanged  and  (II)  those  which  lose  their  identity 
and  are  discharged  as  bodies  different  from  those  which  entered. 
The  first  class  includes  the  foods  furnishing  no  energy;  the  second 
those  furnishing  energy. 

Only  a  few  foods  undergo  in  the  body  reactions  which  alter 
their  identity.  They  may  be  regarded  as  already  digested  and,  in 
fact,  when  dissolved,  ready  for  discharge  from  the  body.  They 
are,  however,  useful  and  necessary  constituents  of  the  body,  and 
if  they  do  not  take  a  considerable  active  part  in  nutrition,  their 
favorable  influence  on  that  process  makes  them  essential  to 
health.  The  foods  producing  no  energy  may  be  dismissed  with 
a  repetition  of  the  statement  that  they  are  largely  introduced  in 
connection  with  the  proteid  foods  from  which  they  cannot  be 
separated  without  destruction  of  the  proteid  molecule.  Indeed, 
all  the  proteid  food  introduced,  whether  animal  or  vegetable, 
contains  inert  constituents  as  a  part  of  the  molecule,  and  these 
seem  as  necessary  to  nutrition  as  do  the  energy  furnishing  con- 
stituents. The  foods  furnishing  energy  and  those  furnishing  no 
energy  enter,  are  deposited,  and  seem  to  be  discharged  both 
together.  The  few  reactions  which  the  inert  foods  undergo  in 
the  body  do  not  materially  affect  the  supply  of  energy. 

(II)  The  proteids,  carbohydrates  and  hydrocarbons  are  all  con- 
sumed in  the  organism,  none  (unless  they  have  accidentally 
escaped  digestion)  being  discharged  as  they  entered. 

i.  The  nitrogenous  foods  are  changed  into  peptones  in  the 
alimentary  canal,  undergo  some  unknown  change  in  their  ab- 
sorption therefrom,  appear  in  the  blood  as  the  proteid  constitu- 


174  NUTRITION,    DIETETICS  AND  ANIMAL   HEAT 

ents  of  that  fluid,  and  are  offered  to  the  tissues  through  the  me- 
dium of  the  lymph.  The  complex  proteid  molecule  is  broken 
down  into  simpler  but  more  stable  ones.  These  end  products 
are  carbon  dioxide,  water  and  urea,  together  with  some  sul- 
phates and  phosphates,  the  production  of  which  is  comparatively 
immaterial.  The  urea  is  distinctive.  Heat,  which  is  equivalent 
to  so  much  energy,  is  evolved  in  the  oxidation  process. 

It  is  probable  that  not  all  the  proteid,  under  the  ordinary  diet, 
is  actually  built  up  into  cell  substance.  A  part  of  it  seems  to 
be  destroyed  without  being  transformed  into  protoplasmic  ma- 
terial, but  the  destruction  always  takes  place  through  the  agency 
of  the  cells,  and  the  end  products  are  always  the  same,  whether 
disassimilation  of  the  proteid  occurs  with  or  without  its  becoming 
an  intrinsic  part  of  the  cell. 

Nitrogenous  Equilibrium — Circulating  and  Tissue  Proteids. — 
The  fact,  however,  that  the  characteristic  function  of  the  ni- 
trogenous foods  is  to  furnish  protoplasmic  material  should  not 
be  lost  sight  of.  A  certain  amount  is  necessary  to  maintain  "ni- 
trogenous equilibrium";  that  is,  to  keep  the  intake  of  nitrogen  up 
to  the  output.  When  nitrogenous  food  is  withdrawn  there  con- 
tinues to  be  a  discharge  of  urea,  which  is  the  chief  nitrogenous 
excretion  and  the  amount  of  which  represents  the  amount  of 
nitrogenous  disassimilation  in  the  body.  The  urea  eliminated 
under  these  conditions  must  represent  the  actual  destruction  of 
cell  substance,  and,  since  the  supply  is  zero  and  the  output  is 
considerable,  there  is  not  a  state  of  nitrogenous  equilibrium;  the 
animal  is  suffering  destruction  of  its  protoplasm  without  a  com- 
pensatory constructive  process.  On  the  other  hand,  the  supply 
of  nitrogenous  material  may  be,  and  usually  is,  in  excess  of  the 
demands  of  the  cells  for  the  actual  regeneration  of  their  substance. 
This  excess  may  be  termed  "  circulating  proteid"  and  is  that  just 
referred  to  as  being  oxidized  under  the  influence  of  the  cells,  but 
without  being  transformed  into  protoplasm.  That  part  of  the 


FOODS   IN   NUTRITION  175 

nitrogenous  supply  which  is  built  up  into  a  part  of  the  cell  has 
been  called  "tissue  proteid"  Whether  any  given  molecule  of 
proteid  food  pass  through  the  system  as  circulating  or  tissue 
proteid  is  only  an  accident — provided  the  supply  be  above  the 
demand  of  the  cells  for  tissue  proteid;  these  demands  are  the 
first  to  be  supplied  by  the  nitrogenous  material  at  hand. 

From  this  it  is  not  to  be  inferred  that  the  exigencies  of  nutrition 
will  be  met  as  well  without  as  with  circulating  proteid.  When 
the  diet  consists  of  just  enough  proteid  to  supply  the  tissue  wastes 
and  of  ample  carbohydrate  and  hydrocarbon  materials,  the  nu- 
tritive process  is  impaired.  It  seems  necessary  to  perfect  health 
that  the  supply  of  nitrogenous  food  be  sufficient  to  allow  for 
the  oxidation  of  some  of  it  as  circulating  proteid  in  a  manner 
analogous  to  oxidation  of  the  non-nitrogenized  materials.  Life 
can  be  maintained  on  nitrogenous  food  alone,  but  it  is  obvious 
that  when  this  is  done  the  amount  of  circulating  proteid  must 
be  enormously  increased  so  that  it  may  be  oxidized  to  furnish 
energy  for  the  body;  for  those  substances,  the  oxidation  of 
which  corresponds  to  oxidation  of  the  circulating  proteids  and 
which  furnish  the  main  supply  of  energy  for  doing  work  (viz., 
the  carbohydrates  and  hydrocarbons),  are  now  withdrawn  from 
the  economy.  It  follows,  conversely,  that  the  ingestion  of  car- 
bohydrates and  hydrocarbons  lessens  the  amount  of  proteid 
necessary  to  nutrition. 

The  albuminoids,  such  as  gelatin  (not  meant  to  be  included 
under  the  term . " nitrogenous"  foods,  though  they  contain  nitro- 
gen), cannot  take  the  place  of  tissue  proteid;  they  may  be  burnt 
in  lieu  of  the  circulating  proteids  and  supply  energy  just  as  the 
carbohydrates  and  fats  do. 

It  is  to  be  remembered  that  any  excess  of  proteid  or  albumin- 
oid food  is  not  discharged  as  such  in  the  excreta,  but  undergoes 
oxidation,  the  end  products  of  which  are  always  the  same,  water, 
carbon  dioxide  and  urea,  or  related  substances;  the  develop- 


176  NUTRITION,    DIETETICS  AND  ANIMAL  HEAT 

ment  of  heat  is  also  an  invariable  accompaniment  of  their 
destruction. 

While  a  person  may  live  on  proteid  food,  the  amount  neces- 
sary taxes  the  digestive  and  excretory  organs  to  such  an  extent 
that  life  is  probably  shortened.  Since  the  total  amount  of  urea 
is  discharged  by  the  kidney,  that  organ,  under  an  excess  of  pro- 
teid diet,  is  particularly  prone  to  degenerative  changes  of  a  most 
serious  nature. 

2.  The  carbohydrates  enter  the  blood  from  the  alimentary 
canal  as  dextrose,  are  conveyed  to  the  liver  and  converted  into 
glycogen,  which  is  stored  up  there  to  be  dealt  out  to  the  blood 
gradually,  after  being  reconverted  into  dextrose.  Dextrose 
exists  in  the  blood  for  a  short  time  only,  being  converted  into 
other  substances,  but  its  final  oxidation  is  effected  by  the  tissues. 
Its  end  products  are  carbon  dioxide  and  water,  with  heat. 
Sugar  (dextrose)  injected  into  the  blood  soon  disappears.  It  is 
thought  by  some  to  be  converted  into  alcohol  in  the  blood  and 
then  oxidized.  At  any  rate,  the  formation  of  the  end  products 
just  mentioned  is  the  final  fate  of  the  carbohydrates,  through 
whatever  splitting  processes  the  sugar  molecule  may  pass  before 
it  is  converted  into  these  substances. 

The  removal  of  the  pancreas  occasions  diabetes  mellitus,  and 
the  inference  is  that  this  gland  gives  off  to  the  blood  some  internal 
secretion  which  splits  up  the  sugar  molecule  in  the  blood.  How 
this  lesion  causes  the  disease  in  question  is  not  clear,  but  the 
retention  of  a  small  part  of  the  gland  enables  the  oxidation  of 
sugar  by  the  tissues  to  proceed  in  the  proper  way  and  it  is  not 
discharged  in  the  urine. 

Value  of  the  Carbohydrates  in  Nutrition. — The  distinctive 
function  of  the  carbohydrates  is  to  act  as  fuel  for  the  body  ma- 
chine; they  are  burnt  up  to  supply  heat,  and  heat  represents 
energy.  Hydrogen  and  oxygen  exist  already  in  the  proportion 
to  form  water — one  of  the  end  products — and  only  enough  O 
is  required  to  unite  with  the  carbon  of  the  carbohydrates  to  form 


FOODS   IN   NUTRITION  177 

CO2 — the  other  end  product.  The  burning  (oxidation)  of  a 
carbohydrate  outside  the  body  results  in  the  formation  of  CO2 
and  H2O  and  the  elimination  of  heat,  which  last,  if  properly 
utilized,  can  be  converted  into  energy — the  power  to  do  work. 
The  result  of  the  oxidation  of  a  carbohydrate  in  the  body  is  the 
same.  Since  this  class  of  food  is  easily  handled  by  the  alimen- 
tary canal,  requires  little  extra  O  for  its  destruction,  and  is  very 
abundantly  supplied  by  the  vegetable  world,  it  is  the  most  eco- 
nomical from  digestive,  absorptive,  respiratory  and  financial 
standpoints.  Carbohydrates  may  also  be  deposited  as  adipose 
tissue  as  will  be  seen  presently. 

3.  The  fats  have  the  same  general  office  in  nutrition  as  the 
carbohydrates,  viz.,  the  furnishing  of  energy  by  their  oxidation. 
They  leave  the  alimentary  canal  by  way  of  the  lacteals,  are 
conveyed  by  the  blood  to  the  tissues  and  there  oxidized  with 
the  formation  of  carbon  dioxide  and  water  and  the  liberation 
of  heat.  Though  more  O  is  necessary  to  burn  up  the  fat  than 
the  carbohydrate  molecule,  oxidation  of  the  fat  is  attended  with 
the  liberation  of  the  greater  amount  of  heat — i.  e.,  of  energy. 
This  would  seem  to  indicate  that  it  would  be  more  economical 
to  eat  fats  to  the  exclusion  of  carbohydrates,  since  a  smaller 
quantity  of  the  former  will  supply  the  requisite  amount  of  energy. 
This  is  theoretically  true,  but  considerations  of  digestion  render 
it  not  practically  so,  since  fats  tax  the  digestive  apparatus  much 
more  than  carbohydrates. 

The  fat  deposited  in  the  body — the  adipose  tissue — whatever 
may  be  its  source,  is  to  be  looked  upon  as  so  much  stored-up 
energy.  When  the  supply  of  blood  is  cut  off  it  is  the  first  part 
of  the  organism  to  be  consumed.  Hence,  a  fat  animal  will  survive 
starvation  longer  than  a  lean  one. 

The  individuality,  the  functional  activity,  and  the  properties 
involved  in  regeneration  of  protoplasm  are  ultimately  depend- 
ent upon  its  nitrogenous  characters.  The  other  constituents  are 
more  or  less  passive.  The  oxidation  of  fats  and  carbohydrates, 


1 78  NUTRITION,    DIETETICS  AND  ANIMAL  HEAT 

however,  takes  place  under  the  influence  and  through  the  agency 
of  the  cells.  It  is  scarcely  necessary  to  add  that  neither  fats  nor 
carbohydrates,  nor  both  together,  are  sufficient  to  sustain  life; 
for  life  is  embodied  in  protoplasm  and  protoplasm  must  have 
nitrogen,  which  element  these  foods  cannot  furnish. 

Formation  of  Adipose  Tissue. — The  adipose  tissue  in^the 
body  is  not  the  result  of  direct  deposition  of  the  oleaginous  foods. 
The  amount  of  fat  taken  on  in  a  given  time  by  some  animals,  as 
hogs,  is  often  far  in  excess  of  the  quantity  of  fat  in  the  ingesta. 
Adipose  tissue  is,  under  normal  conditions,  the  result  always  of 
changes  due  to  protoplasmic  activity.  It  is  formed  by  the  tissues 
chiefly  from  the  carbohydrates,  but  also  to  a  less  extent  from 
the  proteids.  The  chemical  changes  by  which  sugar  is  con- 
verted into  fat  are  as  yet  undetermined,  but  there  are  so  many 
evidences  of  an  increase  in  body  fat  upon  an  excess  of  carbo- 
hydrate food  that  the  fact  itself  that  this  class  of  food  is  the 
main  source  of  fat  is  no  longer  disputed. 

As  regards  the  formation  of  fat  from  proteids,  it  is  thought  that 
the  molecule  is  split  up  into  a  nitrogenous  molecule,  which  is 
discharged  as  urea,  and  a  non-nitrogenous,  which  at  once,  or 
after  undergoing  other  changes,  is  deposited  as  fat.  Experi- 
mental observations  demonstrate  that  the  liver  produces  gly- 
cogen  on  a  purely  proteid  diet.  Since  glycogen  is  a  carbohy- 
drate, and  carbohydrates  are  the  chief  source  of  body  fat,  it  is  not 
improbable  that  the  non-nitrogenous  molecule  of  the  proteid 
dissociation  takes  the  form  of  glycogen  and  is  later  converted 
into  fat  after  the  manner,  whatever  it  may  be,  of  the  glycogen 
introduced  in  carbohydrate  form.  When  the  carbon  discharged 
is  less  than  the  carbon  ingested  the  deficit  is  thought  to  be  retained 
to  form  fat,  which  is  deposited  as  a  reserve  to  be  used  whenever 
its  oxidation  may  become  necessary  as  a  supply  of  energy. 

It  follows  that  to  reduce  body  fat  the  carbohydrates  should  be 
largely  interdicted,  while  to  increase  it  they  should  be  taken  in 
excess.  In  human  beings  proper  regulation  of  the  diet  is  more 


CONDITIONS   INFLUENCING   METABOLISM  179 

efficacious  in  reducing  than  increasing  the  amount  of  adipose 
tissue. 

Adipose  Tissue  a  Reserve  Supply  of  Energy. — The  carbohy- 
drates and  fats  are  preeminently  the  energy-producing  foods,  and 
of  these  the  carbohydrates,  for  reasons  indicated,  are  the  more 
important.  They  not  only  furnish  energy  which  is  immediately 
used  up  in  running  the  machinery  of  the  body,  but  they  deposit, 
or  attempt  to  deposit,  a  reserve  supply  to  protect  the  proteid  por- 
tions of  the  organism  against  accidents  to  temporary  deprivation 
of  food,  demands  for  an  unusual  amount  of  energy,  malnutrition 
from  various  causes,  etc. — sayings  laid  for  the  proverbial  rainy 
day.  This  reserve  supply  takes  the  form  first  of  glycogen,  which 
is  soon  used  up,  meeting  as  it  were  only  the  demands  of  the  hour, 
and  second  of  fat,  which  begins  to  be  drawn  upon  when  the  glyco- 
gen is  exhausted,  and  which  lasts  for  a  length  of  time  depending 
upon  its  amount. 

Conditions  Influencing  Metabolism. — Regular  exercise  is 
undoubtedly  favorable  to  the  nutrition  of  any  part,  as  e.  g.,  the 
muscles,  the  brain,  etc.  Exercise  may  mean  increased  disas- 
similation,  but  if  so  it  also  means  increased  assimilation.  With 
regard  to  muscular  exercise  of  average  severity  and  reasonable 
duration,  the  results  of  cellular  activity  seem  at  first  a  little  sur- 
prising, but  are  really  to  be  expected  if  the  concluding  remarks 
of  the  previous  paragraph  are  true.  The  amount  of  urea  under 
such  exercise  is  not  appreciably  increased — which  means  that 
disassimilation  in  the  protoplasm  of  the  muscle  cells  is  not  in- 
creased. This  remark  holds  good,  however,  only  when  the 
supply  of  sugars,  starches  and  fats  is  abundant;  if  they  are  not 
present  in  sufficient  quantity  to  meet  the  increased  demand  for 
energy-supplying  materials,  then  the  proteids  must  be  oxidized  to 
furnish  it,  and  the  urea  discharged  is  increased.  In  striking  con- 
trast to  the  constant  output  of  urea  is  the  largely  increased  out- 
put of  CO 2,  representing  oxidation  of  the  carbohydrates  and 
fats. 


l8o  NUTRITION,    DIETETICS  AND   ANIMAL  HEAT 

During  sleep  the  nitrogenous  output  is  not  materially  dimin- 
ished, while  that  of  CO2  is  markedly  less.  This  is  explained  by 
the  fact  that  there  is  less  energy  needed  and  correspondingly 
less  oxidation  of  the  energy-producing  materials.  Proteid 
metabolism  is  undisturbed. 

A  low  external  temperature  does  not  increase  the  output  of 
urea;  it  increases  the  output  of  CO2.  These  two  facts  together 
mean  again  that  only  the  carbohydrates  and  fats  are  being  oxi- 
dized in  increased  amount.  This  increased  oxidation,  the  effect 
of  which  is  to  maintain  the  normal  body  temperature  is  usually 
dismissed  with  the  statement  that  it  is  a  reflex  nervous  act.  It  is 
claimed  by  Johannson  that  the  CO2  output  is  not  increased  until 
shivering  occurs  (Reichert).  That  being  the  case,  the  increase  is 
explained  on  the  ground  of  increased  energy  and  heat  production 
incident  to  muscular  exercise,  and  shivering  assumes  the  dignity 
of  a  physiological  factor  in  keeping  up  the  temperature  of  the 
body.  This  is  perfectly  reasonable  when  it  is  remembered  how 
effective  active  muscular  exercise  is  in  keeping  the  body  warm. 
But  the  fact  that  a  person  when  cold  shivers  and  is  restless  invol- 
untarily does  not  allow  us  to  escape  the  unsatisfactory  "reflex 
action"  explanation  of  the  phenomenon  in  question.  Within 
ordinary  and  reasonable  limits  proteid  metabolism  is  undis- 
turbed; it  is  still  being  protected  by  the  fats  and  carbohydrates. 

During  starvation  nothing  is  supplied  from  the  outside  world 
except  oxygen,  and  the  animal  must  live  on  the  materials  al- 
ready in  his  body.  The  glycogen  is  first  consumed;  it  is  the 
surplus  on  hand;  but  at  best  it  is  all  gone  in  a  very  few  days. 
Then  the  fat  stored  up  as  adipose  tissue  is  drawn  upon;  it  is  the 
reserve  fund;  but  it  is  likewise  soon  consumed;  the  animal  be- 
comes progressively  emaciated.  When  this  is  exhausted  the 
tissue  proteid  is  attacked;  this  is  the  capital  and  is  the  last  to  be 
touched;  but  there  must  be  heat  and  at  least  some  energy,  and 
there  is  no  other  source.  When  the  proteid  capital  has  at  last 
been  so  impaired  that  it  can  no  longer  furnish  heat  to  maintain 


REQUISITES    OF   DIET  I&I 

the  body  temperature  and  energy  to  carry  on  the  necessary  or- 
ganic functions,  the  organism  is  physiologically  bankrupt  and 
assignment  follows — death  is  at  hand. 

DIETETICS. 

The  appetite,  under  normal  conditions,  may  be  depended  upon 
to  regulate  both  quantity  and  quality  of  diet  in  a  fairly  satisfactory 
manner.  Different  peoples  require  different  proportions  and 
amounts  of  the  various  food-stuffs  and  the  same  is  true  of  any 
given  individual  for  varying  conditions  of  temperature,  exercise, 
etc.  But  in  any  case  the  object  of  eating  is  to  prevent  the  loss, 
in  aggregate,  of  proteid  tissue,  fat,  etc. — to  replace  the  wastes, 
and  that  in  the  most  convenient  and  economical  way. 

When  the  ingesta  exceed  the  excreta  the  animal  is  gaining  in 
weight;  when  opposite  conditions  obtain  he  is  losing;  when 
there  is  a  balance  between  the  two  the  body  equilibrium  is  being 
maintained. 

Determination  of  the  Requisites  of  a  Diet. — The  usual 
method  of  determining,  in  a  scientific  manner,  the  requisites  of  a 
normal  diet  for  persons  in  general  is  to  estimate  the  amount  of 
the  various  excretions  from  the  bodies  of  a  limited  number  of  per- 
sons in  health,  and  from  this  knowledge  to  calculate  the  amount 
and  kind  of  food  which  will  supply  the  demands  in  the  most 
satisfactory  way,  it  being  assumed  that  these  excretions  represent 
the  normal  and  necessary  metabolism  going  on  in  the  body.  The 
results  of  such  examination  are  found  to  correspond  with  the 
actual  demands  of  the  system. 

It  has  been  seen  that  the  organism  demands  some  fifteen  or 
more  chemical  elements  for  use  to  keep  itself  in  good  running 
order;  it  has  been  seen  also  that  its  demands,  so  far  as  quantity 
is  concerned,  are  chiefly  confined  to  carbon,  hydrogen,  oxygen 
and  nitrogen.  The  other  elements  deserve  no  attention  here 
since  they  (excepting  sodium  chloride)  are  unconsciously  intro- 
duced with  the  ordinary  foods  in  amounts  sufficient  to  satisfy  the 


1 82  NUTRITION,    DIETETICS  AND  ANIMAL  HEAT 

requirements  of  the  system.  Moreover,  the  air  we  breathe  and 
the  water  we  drink  furnish  an  ample  supply  of  hydrogen  and 
oxygen  when  to  this  supply  is  added  the  quota  of  these  elements 
contained  in  the  necessary  quantities  of  other  aliments.  So, 
therefore,  if  we  fix  upon  a  diet  which  will  furnish  the  requisite 
amounts  of  carbon  and  nitrogen  no  attention  need  be  paid  to 
the  other  elements.  The  supply  of  the  others  may  be  said  to 
regulate  itself  if  the  supply  of  carbon  and  nitrogen  be  regulated. 

The  object,  then,  of  food  may  be  said  to  be  the  replacement 
of  carbon  and  nitrogen — the  carbon  and  nitrogen  in  the  excreta. 
Of  these  two  elements,  carbohydrates  and  fats  will  furnish  only 
carbon;  proteid  food  will  furnish  both. 

Amount  of  C  and  N  Necessary.— It  is  found  that  the  daily 
discharge  of  nitrogen  is  about  18  grams  (4i3)  an<i  of  carbon 
about  281  grams  (8J§).  These  are  the  amounts,  therefore, 
which  must  be  supplied  by  food.  We  may  accept,  as  represent- 
ing the  proteid  molecule  in  general,  the  formula,  C72H112O22N18S. 
Then  it  is  evident  that  an  amount  of  proteid  food  which  would 
furnish  the  necessary  18  grams  of  nitrogen  would  furnish  only  72 
grams  of  carbon — only  about  one-fourth  enough.  If,  now,  the 
proteid  food  be  increased  to  supply  281  grams  of  carbon,  the  sys- 
tem will  have  to  handle  four  times  as  much  nitrogen  as  it  needs; 
and  this  is  a  tax  to  the  digestive  apparatus  and  the  excretory 
organs,  particularly  the  kidney — a  tax  which  is.  rendered  unnec- 
essary by  the  availability  of  the  carbohydrates  and  fats  as  food. 
These  contain  abundance  of  carbon,  and  it  is  far  better  to  eat 
only  enough  proteid  food  to  supply  the  18  grams  of  nitrogen,  and 
make  up  the  deficit  of  carbon  with  non-nitrogenized  articles  of 
diet.  One  can  supply  all  the  demands  by  eating  nitrogenous  food 
alone,  and  life  will  be  preserved  indefinitely  perhaps,  but  the  pre- 
diction would  be  warranted  that  in  such  a  case  the  person  would 
probably  die  prematurely — as  a  result  of  kidney  or  liver  disease. 

Articles  Which  will  Supply  the  Necessary  Amounts  of 
C  and  N. — The  conclusion  (modified)  of  Moleschott  is  that 


REQUISITES    OF    DIET 


the  average  man  needs  daily  about  120  grams  of  proteid,  90 
grams  of  fat,  and  320  grams  of  carbohydrate  food,  estimated  dry; 
and  that  with  this,  in  the  usual  state  in  which  such  food  is  taken, 
he  will  consume  unconsciously,  or  as  a  result  of  craving,  some 
30  grams  of  salts  and  2,800  grams  of  water.  These  proportions 
are  supposed  to  satisfy  the  demands  of  the  system  in  an  econom- 
ical way.  The  estimates  of  Ranke  vary  somewhat  from  this  as 
indicated  in  the  subjoined  table  which  shows  also  the  balance 
kept  up  in  the  body. 


Income. 

Expenditure. 

Foods. 

Nitrogen. 

Carbon. 

Excretions. 

Nitrogen. 

Carbon. 

Proteid,  100  gm  . 
Fat,  100  gm 

15-5  gm. 
o.o    " 

53-o  gm. 
79.0    ' 

Urea,  31.5  gm. 
Uric  acid,  0.5 

j.4.4 

6.16 

Carbohydrates, 

o.o'  " 

93-0    " 

gm. 

250  gm. 

Feces 

I.I 

10.84 

Respiration 

O.O 

208.00 

(C02) 

15.5  gm. 

225.0  gm. 

J5-5 

225.00 

The  actual  amounts  of  given  substances  which  it  is  necessary 
to  eat  in  order  to  supply  the  requirements  of  these  estimates 
depend,  of  course,  on  the  composition  of  those  substances,  and 
would  have  to  be  settled  by  reference  to  a  table  giving  analyses 
of  the  common  articles  of  diet.  Two  pounds  of  bread  and  J 
pound  (when  uncooked)  of  lean  meat,  together  with  water  and 
salt,  will  supply  the  demands;  but  this  is  an  unusual  diet.  Or  i 
pound  of  meat,  i  pound  of  bread  and  ^  pound  of  butter,  or  other 
fat,  with  water  and  salt  is  probably  preferable. 

In  any  case  if  nutrition  is  to  be  properly  performed  the  diet 
must  be  varied.  It  could  not  be  held  that  the  above  supply  of 


184  NUTRITION,    DIETETICS  AND  ANIMAL  HEAT 

food  would  keep  a  person  indefinitely  in  good  health.  His  de- 
mands for  nitrogen  and  carbon  are  always  approximately  the 
same,  but  the  organism  revolts  at  being  supplied  with  them  from 
exactly  the  same  source  for  any  considerable  length  of  time. 
As  a  diet  is  necessary  (Schenck  and  Gurber): 

Proteid.  Fat.  Carbohydrates. 

Resting  man 100  gm.  60  gm.         400  gm. 

Resting  woman 90  gm.  40  gm.         350  gm. 

Working  man 130  gm.  100  gm.         500  gm. 

It  need  scarcely  be  added  that  any  condition,  such  as  exer- 
cise, temperature,  etc.,  which  increase  the  excreta,  calls  for  a 
larger  supply  of  ingesta.  Ordinary  exercise  is  allowed  for  in 
the  estimates  just  given. 

ANIMAL  HEAT. 

The  Temperature. — The  average  temperature  of  the  human 
body,  taken  under  the  tongue,  is  98.5°  F.  It  varies  in  different 
parts,  the  mean  being  about  100°.  The  metabolic  activity  in 
different  parts  of  the  body  is  changeable,  and  consequently  the 
heat  production  in  all  parts  is  not  the  same. 

The  fact  that  the  temperature  is  nearly  identical  throughout 
the  body  is  due  to  the  distribution  of  heat,  which  distribution  is 
mainly  effected  through  the  agency  of  the  circulating  fluids. 
The  rectal  temperature  is  a  little  higher  than  that  obtained  in 
the  mouth.  The  temperature  of  arterial  is  higher  than  that  of 
venous  blood.  The  warmest  blood  is  in  the  hepatic  veins;  the 
coolest  is  that  which  has  just  passed  through  the  most  exposed 
peripheral  parts,  as  the  helix  of  the  ear.  The  mean  body  tem- 
perature is  a  little  lower  in  the  morning  than  in  the  evening,  in 
the  female  than  in  the  male,  on  a  restricted  than  on  an  abun- 
dant diet,  in  cold  than  in  hot  climates,  and,  in  general,  in  condi- 
tions of  diminished  than  of  exalted  metabolic  activity. 

But  in  health  these  variations  are  of  trivial  importance  and  do 


HEAT  AND    FORCE  185 

not  represent  a  sweep  of  more  than  2°  F.  The  body  temper- 
ature may  be  looked  upon  as  being  a  fairly  constant  quantity. 
It  varies  scarcely  at  all  with  variations  of  external  temperature, 
so  long  as  the  heat-  regulating  apparatus  is  in  order.  An  external 
(dry)  temperature  of  212°  F.,  or  the  extremely  low  temperature 
of  some  regions,  can  be  borne  with  very  slight  fluctuations  in 
that  temperature  of  the  body.  The  actual  limits  of  internal  tem- 
perature consistent  with  the  preservation  of  life  are  given  by 
Flint  as  83°  and  107°  F.  These  temperatures  cannot  be  long 
endured. 

The  fundamental  fact  to  be  kept  constantly  in  mind  is  that 
there  is  a  continual  production  and  a  continual  dissipation  of 
heat,  in  ways  to  be  indicated  presently.  These  two  processes 
are  known  as  thermo genesis  (heat  production)  and  thermolysis 
(heat  loss)  respectively.  The  preservation  of  the  proper  balance 
between  heat  production  and  heat  dissipation  is  known  as 
ihermotaxis. 

Supply  of  Heat  and  its  Relation  to  Force. — It  is  a  matter  of 
common  observation  that  the  burning  (oxidation)  or  any  sub- 
stance, as  a  piece  of  wood  or  an  article  of  diet,  is  accompanied 
by  the  evolution  of  heat.  It  is  also  known  that  heat  may  be 
converted  into  force — may  be  made  to  do  work.  The  burning 
of  a  fat  or  a  sugar  produces  CO2  and  H2O ;  the  burning  of  a  pro- 
teid  produces  CO2  and  H2O,  and  additional  substances.  The 
final  products,  and  the  amount  of  heat  evolved,  are  precisely  the 
same  whether  the  oxidation  be  rapid  or  slow.  Now,  the  oxida- 
tion of  food  is  exactly  what  occurs  in  the  human  organism,  though 
that  of  the  proteids  is  not  completely  effected;  CO2  and  H2O  are 
produced  from  them,  and  the  " additional  substances"  men- 
tioned are  represented  by  urea.  This  process,  then,  is  the  source 
of  body  heat.  To  the  supply  thus  furnished  may  be  added  a 
little  from  reactions  between  non-energy  producing  materials  in 
the  body,  from  warm  foods  and  drinks,  and  from  friction  in  the 
vessels,  joints,  etc. 


1 86  NUTRITION,    DIETETICS  AND  ANIMAL  HEAT 

The  foods  thus  possess  a  certain  potential  energy,  an  energy 
which  may  be  converted  directly  or  indirectly  into  heat,  or  its 
equivalent.  The  potential  energy  of  the  foods  keeps  up  the 
body  temperature  and  supplies  force  for  doing  work.  It  is  con- 
verted into  heat  and  kinetic  energy.  Kinetic  energy  is  working 
energy,  and  is  represented  in  the  body  chiefly  by  muscular  con- 
tractions. But,  since  this  kinetic  energy  has  its  source  in  the 
transformation  of  food  stuffs,  and  since  kinetic  energy  and  heat 
are  mutually  convertible,  it  may  be  assumed  that  all  the  potential 
energy  of  the  foods  is  converted  into  heat.  The  kinetic  energy 
may  be  taken  as  representing  so  much  heat,  and  the  total  pro- 
duction of  heat  (including  kinetic  energy)  as  representing  the 
total  production  of  energy.  Or,  to  state  the  case  differently  the 
potential  energy  of  the  food  is  converted  into  heat,  a  part  of 
which  appears  as  kinetic  energy.  By  far  the  largest  part  of  this 
potential  energy,  however,  is  converted  directly  into  heat.  Not 
more  than  one-fifth  of  the  heat  produced  in  the  body  can  be 
utilized  to  do  work,  and  a  part  of  that  work  is  actually  converted 
indirectly  into  heat,  and  contributes  to  the  total  heat  of  the 
body,  by  overcoming  friction  incident  to  respiration,  circulation, 
movements  of  the  joints,  muscles,  etc. 

Potential  Value  of  Foods. — It  is  estimated  that  the  oxidation 
in  the  body  of  one  gram  of  fat  produces  9,300  calories  of  heat, 
one  gram  of  carbohydrate  4,100  calories,  and  one  gram  of  proteid 
4,100  calories.  These  figures  represent  the  potential  energy  of 
the  several  foods.  Fats,  it  is  seen,  produce,  weight  for  weight, 
more  than  twice  as  much  energy  as  other  foods,  but  reasons  have 
been  given  why  they  cannot  be  used  exclusively. 

A  calorie  is  the  amount  of  heat  necessary  to  raise  i  Kg  of 
water  from  o°  to  i°  C.  A  grammeter  is  the  amount  of  energy 
necessary  to  raise  i  gram  i  meter.  Now  since  heat  and  work 
are  only  different  forms  of  energy,  these  two  units — calorie  and 
grammeter — have  each  equivalents  in  terms  of  the  other.  One 
calorie  equals  424.5  grammeters;  that  is,  the  force  represented  by 


TOTAL  AND    SPECIFIC  HEAT  187 

one  calorie  will  raise  one  gram  424.5  meters.  The  terms  kilo- 
calorie,  or  kilogramdegree,  and  kilogrammeter  are  used  some- 
times, and  represent  1,000  times  the  calorie  and  grammeter 
respectively. 

Total  and  Specific  Heat. — The  temperature  of  a  body  indi- 
cates nothing  as  to  the  quantity  of  the  heat  it  contains.  The 
degree  of  heat  requires  only  a  thermometer  to  determine  it,  but 
the  quantity  depends  on  the  temperature,  the  weight  and  the 
specific  heat  of  the  substance  in  question. 

Specific  heat  is  analogous  to  specific  gravity.  Water  is  taken  as 
the  standard  in  both  cases.  If  it  require  only  .5  calorie  to  raise 
i  gram  of  a  certain  substance  i  degree  C.,  the  specific  heat  of  that 
substance  is  said  to  be  .5.  The  specific  heat  of  the  body  is  .8; 
that  is,  whereas  it  requires  a  certain  amounnt  of  heat  to  raise 
150  pounds  of  water  to  a  certain  temperature,  it  would  require 
only  .8  as  much  to  raise  a  human  body  weighing  the  same  to 
the  same  temperature.  To  find  the  total  heat  in  calories  in  any 
body  it  is  only  necessary  to  multiply  the  weight  (in  grams)  by 
the  specific  heat  and  by  the  temperature  C.  Estimates  made 
by  calorimetry  from  these  data  and  from  the  potential  value  of 
the  different  foods  give  the  total  daily  heat  production  as  about 
2,500,000  calories  for  the  average  individual.  This  is  equal  to 
about  1,400  calories  per  hour  per  kilo  weight. 

The  English  heat  unit  is  the  pound-degree  F.  It  is  the  amount 
of  heat  necessary  to  raise  i  pound  of  water  i  degree  F.  Its 
mechanical  equivalent  is  the  force  necessary  to  raise  i  pound 
772  feet.  The  estimates  just  given  in  the  metric  system  when 
translated  to  English  nomenclature  give  the  total  heat  produc- 
tion for  24  hours  as  about  8,400  pound-degrees,  or  2.5  per  hour 
per  pound  weight.  These  figures  are  given  as  only  approx- 
imate and  are  subject  to  change  by  many  causes,  such  as  sex, 
cardiac  and  respiratory  activity,  internal  and  external  temper- 
ature, exercise,  digestion,  age,  nervous  influences,  the  body 
weight,  etc. 


1 88  NUTRITION,    DIETETICS  AND  ANIMAL  HEAT 

Thermogenesis. — Thermogenesis,  or  the  production  of  heat, 
is  the  result  of  activity  on  the  part  of  tissues,  nerves  and  centers. 
Now,  the  potential  energy  of  the  food  stuffs  is  the  ultimate 
source  of  all  bodily  heat  no  matter  how  it  may  be  manifested, 
and  it  is  evident  from  what  has  been  said  already  that  all  the 
tissues  of  the  body  are  heat-producing  tissues,  because  oxidation 
processes  go  on  in  them  all.  But  muscular  tissue  seems  to  be 
endowed  with  special  heat-producing  capabilities,  so  much  so 
that  it  is  said  to  generate  heat  as  a  specific  product,  and  not  as  a 
mere  incident  of  its  metabolism.  Muscle  will  reproduce  heat 
when  entirely  at  rest — when  the  nutritive  metabolic  changes  are 
practically  nothing.  The  process  seems  to  be  regulated  in  accord- 
ance with  the  needs  of  the  economy  by  means  of  a  nervous  mech- 
anism, making  the  production  of  heat  analogous  to  secretion. 
Separation  of  a  muscle  from  its  nerve  does  not  stop  thermogenesis, 
but  markedly  interfers  with  it  in  that  part.  The  existence  of 
distinct  thermogenic  nerves  has  not  been  demonstrated.  The 
existence  of  specific  thermogenic  centers  seems  certain.  Some  of 
them  increase  and  some  decrease  thermogenesis. 

The  general  thermogenic  centers  are  in  the  spinal  cord.  Cen- 
ters increasing  thermogenesis  are  probably  in  the  caudate  nuclei 
of  the  corpora  striata,  the  optic  thai  ami,  pons  and  medulla. 
Irritation  of  these  regions  causes  a  rise  in  temperature.  The 
location  of  the  thermo-inhibitory  centers  is  a  matter  of  specu- 
lation. The  general  thermogenic  centers  in  the  cord  probably 
maintain  a  fairly  constant  production  of  heat  independently, 
but  they  are  subservient  to  encephalic  centers  which  excite 
them  to  increased  or  decreased  activity  by  reason  of  certain 
impressions,  cutaneous  or  otherwise,  which  they  have  received. 

Heat  Loss. — About  85  per  cent,  of  animal  heat,  discharged 
as  such,  is  lost  by  radiation  and  evaporation  from  the  skin; 
about  12  per  cent,  is  dissipated  in  the  lungs  by  evaporation  and 
in  warming  the  inspired  air;  the  remainder  is  discharged  in  the 


CONDITIONS    INFLUENCING  HEAT    DISSIPATION  189 

urine  and  feces  (disregarding  the  small  amount  which  goes  to 
warm  ingested  articles). 

Heat  is  radiated  from  the  body  just  as  from  a  hot  stove. 
Radiation  is  affected  by  the  conductivity  of  the  surrounding 
medium.  For  instance,  in  media  of  water  and  air  of  the  same 
temperature  the  radiation  is  greater  in  water,  because  it  is  a  better 
conductor  of  heat. 

Evaporation  from  the  skin  is  of  very  great  importance  in  in- 
creasing heat  dissipation.  582  calories  of  heat  are  consumed 
when  one  gram  of  water  is  vaporized ;  and  when  this  evaporation 
takes  place  on  the  skin  the  heat  is  abstracted  largely  from  the 
body.  This  is  said  to  represent  nearly  15  per  cent,  of  the  total 
heat  dissipation.  Hence  the  value  of  perspiring  in  hot  weather. 
Evaporation  also  takes  place  from  the  moist  surfaces  of  the  lungs 
and,  moreover,  when,  as  is  usually  the  case,  the  inspired  air  is 
cooler  than  the  lung  structure,  a  certain  amount  of  heat  is 
consumed  in  warming  it. 

But  it  is  not  to  be  inferred  that  loss  of  heat  takes  place  from 
the  body  just  as  from  an  inanimate  object.  On  the  other  hand, 
it  is  intimately  .connected  with  and  influenced  by  circulation,  res- 
piration, secretion  and  other  functions.  When  there  is  a  tend- 
ency for  the  body  temperature  to  rise,  the  circulation  becomes 
more  active  and  sends  more  blood  to  the  periphery  to  be  cooled; 
respiration  is  augmented,  causing  a  greater  abstraction  in  the 
lungs;  the  secretion  of  sweat,  for  instance,  is  increased. 

There  may  be  distinct  centers  governing  the  loss  of  heat. 

Conditions  Influencing  Heat  Dissipation.— These  have 
been  suggested  in  a  previous  section.  Heat  dissipation  is  greater 
in  proportion  to  weight  in  small  than  in  large  animals  because 
the  radiating  surface  is  relatively  larger.  It  is  less  in  the  female 
than  in  the  male  because  she  has,  as  a  rule,  a  larger  proportion 
of  subcutaneous  fat,  which  is  a  poor  conductor  of  heat.  It  is  less 
when  the  body  is  covered  with  clothing  which  is  a  poor  con- 
ductor of  heat  than  when  the  covering  conducts  heat  readily. 


1 90  NUTRITION,    DIETETICS  AND  ANIMAL  HEAT 

It  is  increased  when  the  internal  temperature  is  raised  and  when 
the  external  temperature  is  lowered.  Any  general  increase 
in  vascular  or  respiratory  activity  increases  heat  dissipation  for 
reasons  already  given.  When  the  external  temperature  is  high 
and  the  air  is  dry  evaporation  is  more  abundant,  and  conse- 
quently heat  dissipation  is  greater  than  when  the  air  is  already 
impregnated  with  moisture.  Hence  the  oppressiveness  of  a 
high  external  temperature  with  high  humidity.  In  fever  heat 
dissipation  is  usually  increased,  but  to  a  less  degree  than  that 
production. 

Thermotaxis. — Thermotaxis  is  the  regulation  of  heat  produc- 
tion and  heat  dissipation  so  that  the  temperature  of  the 'body 
may  remain  the  same.  It  is  evident  that  there  is  frequently  a 
transient  increase  or  decrease  of  thermogenetic  activity;  unless 
there  be  a  corresponding  change  in  thermolytic  activity  the 
temperature  will  be  disturbed. 

The  temperature  of  the  body  is  not  necessarily  raised  when 
heat  production  is  increased,  or  lowered  when  it  is  decreased; 
for  heat  loss  may  be,  and  in  health  is,  correspondingly  increased 
or  diminished.  Conversely,  a  change  in  heat  loss  does  not  nec- 
essarily mean  an  opposite  change  in  the  body  temperature.  Al- 
terations which  do  occur  in  the  temperature  are  the  result  of  the 
improper  regulation  of  the  heat  at  hand.  For  instance,  fever  may 
result  from  average  heat  production  and  deficient  heat  loss; 
from  increased  heat  production  and  heat  loss  when  the  latter  is  in- 
creased less  than  the  former;  from  diminished  heat  production 
and  heat  loss  when  the  latter  is  diminished  less  than  the  former, 
etc.  A  subnormal  temperature  is  caused  by  opposite  condi- 
tions. The  temperature  remains  constant  when  heat  production 
and  loss  are  normal,  or  when  they  are  increased  or  decreased 
correspondingly. 

Thermotactic  activity  is  the  result  of  changes  in  the  tempera- 
ture of  the  blood,  or  of  cutaneous  impressions.  A  rise  in  the  tem- 
perature of  the  blood  excites  heat  loss,  as  indicated.  A  cold 


THERMOTAXIS  IQI 

atmosphere  increases  heat  loss,  but  at  the  same  time  it  makes 
impressions  on  the  cutaneous  nerves  which,  when  carried  to  the 
centers,  excite  heat  production  and  thus  compensation  is  estab- 
lished. A  cold  bath  lowers  the  temperature  because  heat  loss  is 
increased  more  than  heat  production.  There  is  increased  radia- 
tion because  of  the  relatively  increased  difference  in  the  tem- 
perature of  the  body  and  of  the  surrounding  medium.  On  the 
other  hand,  the  cold  contracts  the  capillaries,  diminishing  the 
amount  of  blood  exposed  to  the  cooling  influence  of  the  water 
and  decreasing  the  amount  of  sweat;  but  these  influences  tend- 
ing to  inhibit  heat  loss  are  not  equal  to  those  augmenting  it. 
However,  in  health,  thermotaxis  prevents  the  disturbance  of  the 
balance  between  thermogenesis  and  thermolysis  to  any  great  ex- 
tent, and  the  temperature  cannot  be  lowered  very  much.  These 
are  only  examples  of  the  reciprocal  relations  maintained  between 
the  production  and  dissipation  of  heat,  a  disturbance  of  which 
relations  is  prevented  under  normal  conditions  by  thermotaxis. 
Any  change  in  one  process  is  followed  at  once  by  a  compensatory 
change  in  the  other. 


CHAPTER  X. 
EXCRETION  BY  THE  KIDNEYS  AND  SKIN. 

EXCRETION  of  the  various  foods  after  they  have  discharged 
their  several  functions  in  the  body  is  effected  mainly  by  the 
kidneys,  skin,  lungs  and  alimentary  canal.  The  excretory  action 
of  the  last  two  named  is  considered  under  Respiration  and 
Digestion.  Attention  is  again  called  to  the  fact  that  it  is  impos- 
sible to  differentiate  strictly  between  a  secretory  and  excretory  fluid. 
The  urine  is  as  typical  of  the  excretions  as  any  fluid  to  be  found. 
But  it  will  be  convenient  to  speak  of  the  "secretion"  of  urine 
when  reference  is  made  to  the  act  of  separating  its  constituents 
from  the  blood. 

THE  KIDNEYS. 

Anatomy. — The  kidneys,  one  on  each  side  of  the  body,  are 
behind  the  peritoneum  in  the  lumbar  region.  The  right  is  usu- 
ally a  little  lower  and  a  little  lighter  than  the  left.  The  hilum 
from  which  the  ureter  springs  looks  inward  and  forward.  The 
kidney,  as  found  behind  the  peritoneum,  is  covered  with  a  con- 
siderable amount  of  fat,  but  the  substance  proper  of  the  organ 
is  closely  surrounded  by  a  somewhat  resistant  fibrous  capsule 
which  in  health  can  be  easily  stripped  away.  At  the  hilum  the 
capsule  is  continued  inward  to  line  the  pelvis,  infundibula  and 
calyces. 

The  kidney  belongs  to  the  class  of  compound  tubular  glands. 
If  it  be  cut  into  two  halves  by  an  incision  passing  through  the  two 
borders  (and,  therefore,  through  the  hilum)  an  idea  of  its  gross 
divisions  is  obtained.  The  renal  substance  is  seen  to  be  divided 

192 


STRUCTURE    OF   THE   KIDNEY 


193 


into  an  outer  layer,  known  as  the  cortical  substance,  and  an  inner, 
or  pyramidal,  portion.  Internally  the  incision  reveals  a  cavity 
into  which  the  ureter  opens.  This  is  the  pelvis. 


2" 


FIG.  54. — Longitudinal  section  through  the  kidney,  the  pelvis  of  the  kidney, 
and  a  number  of  renal  calyces.     (From  Brubaker,  after  Tyson.) 

A,  branch  of  the  renal  artery;  U,  ureter;  C,  renal  calyx;  i,  cortex;  i',  medullary 
rays;  i",  labyrinth,  or  cortex  proper;  2,  medulla;  2',  papillary  portion  of  medulla, 
or  medulla  proper;"  2,  border  layer  of  the  medulla;  3,3,  transverse  section  through 
the  axes  of  the  tubules  of  the  border  layer;  4,  fat  of  the  renal  sinus;  5,  5,  arterial 
branches;  *  transversely  coursing  medulla  rays  in  column  of  Bertin. 

Tracing  the  divisions  of  the  pelvis  toward  the  kidney  substance, 
it  is  found  to  be  continued  by  three  short  canals,  one  toward 


194 


EXCRETION   BY   THE   KIDNEYS  AND    SKIN 


the  upper,  one  toward  the  lower  and  one  toward  the  central 
portion  of  the  organ.  These  are  the  three  infundibula.  Each 
infundibulum,  passing  outward,  subdivides  into  two  or  three,  or 
more,  short  cylinder-like  canals  which  receive  the  apices  of  the 


Boundary  or 
I  ^marginal 
zone. 


FIG.  55. 

LS,  of  a  pyramid  of  Malpighi;  PF,  pyramids  of  Ferrein;  RA,  branch  of  renal 
artery  with  an  interlobular  artery;  RV,  lumen  of  a  renal  vein  receiving  an  inter- 
lobular  vein;  VR,  vasa  recta;  PA,  apex  of  a  renal  papilla;  b,  b,  embrace  the  bases  of 
the  lobules.  (Stirling.) 

pyramids.  These  are  the  calyces,  each  of  which  receives  the 
apex  of  one  or  more  pyramids.  The  urine  thus  escaping  from 
the  pyramidal  tubules  passes  in  succession  through  the  calyces, 
infundibula,  pelvis,  and  thence  into  the  ureter. 

The  cortical  substance  constitutes  the  outer  layer  of  the  kid 


STRUCTURE    OF   THE   KIDNEY  195 

neys  and  is  about  J  inch  thick.  It  is  reddish  and  granular  in 
appearance.  From  it  pass  in  between  the  Malpighian  pyra- 
mids columns  known  as  the  columns  of  Berlin.  The  cortical 
substance  contains  the  glomeruli  and  convoluted  tubules  to- 
gether with  blood-vessels  and  lymphatics  supported  by  con- 
nective tissues. 

The  pyramidal  substance,  also  called  the  medullary  sub- 
stance, consists  of  a  number  of  pryamids,  about  12-15,  whose 
bases  look  outward  and  rest  on  the  cortical  substance  and  whose 
apices  look  inward  and  are  received  into  the  calyces.  These 
are  called  the  pyramids  of  Malpighi.  They  contain  uriniferous 
tubules,  vessels,  etc.,  supported  by  connective  tissue.  It  will 
be  seen  that  these  tubes  converge  and  join  each  other  in  passing 
from  the  base  to  the  apex  of  the  pyramid,  so  that  the  very  large 
number  entering  the  base  is  represented  by  only  10-25  a*  ^ne 
apex.  Thus  it  is  that  the  Malpighian  pyramid  is  divided  into  a 
number  of  smaller  pyramids.  These  latter  are  the  pyramids  of 
Ferrein,  and  correspond  in  number  to  the  number  of  tubes  radi- 
ating from  the  apex  of  the  larger  pyramid.  The  medullary 
substance  is  marked  by  striae  which  have  the  direction  of  the 
tubules  and  which  are  caused  by  them.  Its  consistence  is  firmer 
and  its  color  is  darker  than  that  of  the  cortical  substance. 

Malpighian  Bodies. — These  are  scattered  throughout  the 
cortical  substance,  and  are  y-J-o-^ir  inch  in  diameter.  They  con- 
sist of  a  bunch  of  capillaries  in  the  shape  of  a  ball,  the  glomerulus, 
surrounded  by  the  extremity,  or  rather  the  beginning,  of  one  of 
the  renal  tubules.  At  the  point  where  the  tubule  joins  the  Mal- 
pighian tuft  it  is  constricted;  running  then  over  the  glomerulus 
it  reaches  the  afferent  artery  and  the  efferent  vein  on  the  oppo- 
site side;  when  it  has  reached  these  vessels  it  is  reflected  over 
the  whole  network  of  capillaries  so  that  really  the  tuft  is  outside 
the  tube,  but  practically  it  is  covered  by  a  doable  layer  of  the 
tube  wall.  A  space,  the  beginning  of  the  tubule,  is  left  between 
these  two  layers  and  into  it  the  glomerular  secretion  passes.  The 


196 


EXCRETION   BY   THE    KIDNEYS  AND    SKIN 


outer  layer  is  the  capsule  of  Bowman  (or  Miiller) .  Both  layers 
consist  of  a  single  stratum  of  flattened  epithelial  cells;  those  of 
the  inner  layer  are  applied  closely  to  the  glomerulus  and  are 
thought  to  be  very  important  in  secretion.  The  incoming  artery 
breaks  ap  to  form  the  capillary  tuft;  the  corresponding  outgoing 


FIG.  56.  —  Transverse  section  of  a 
developing  Malpighian  capsule  and 
tuft  (human.). 


From  a  fetus  at  about  the  fourth 
month  ;  a,  flattened  cells  growing  to  form 
the  capsule;  b,  more  rounded  cells  con- 
tinuous with  the  above,  reflected  round 
c,  and  finally  enveloping  it;  c,  mass  of 
embryonic  cells  which  will  later  become 
developed  into  blood-vessels.  (Kirkes 
after  W.  Pye.) 


FiG.57. — Epithelial  elements  of 
Malpighian  capsule  and  tuft. 


With  the  commencement  of  a  urinary 
tubule  showing  the  afferent  and  efferent 
vessels;  a,  layer  of  flat  epithelium  form- 
ing the  capsule;  b,  similar,  but  rather 
larger  epithelial  cells,  placed  in  the  walls 
of  the  tube;  c,  cells,  covering  the  vessels 
of  the  capillary  tuft;  d,  commencement 
of  the  tubule,  somewhat  narrower  than 
the  rest  of  it.  (Kirkes  after  W.  Pye.) 


vein  has  a  smaller  caliber  than  the  artery.  The  vein,  having 
left  the  glomerulus,  breaks  up  into  a  secondary  network  around 
the  convoluted  tubes.  This  arrangement  of  the  Malpighian  body 
furnishes  a  most  favorable  opportunity  for  the  passage  of  sub- 
stances out  of  the  blood  current  into  the  beginning  of  the  tube. 


STRUCTURE  OP  THE  KIDNEY  197 

Uriniferous  Tubules. — These  begin  at  the  glomeruli  and  end 
at  the  apices  of  the  Malpighian  pyramids.  From  their  tortuous 
course  in  the  cortical  portion  they  are  there  called  convoluted 
tubules,  in  contradistinction  to  the  straight  tubes  of  the  medullary 
portion.  This,  however,  is  only  a  general  division;  further 
divisions  are  to  be  noted. 

The  constricted  portion  of  the  tube  where  it  leaves  the  glomer- 
ulus  is  the  (i)  neck;  passing  away  from  the  neck  the  tubule  be- 
comes very  tortuous  and  is  known  as  the  (2)  primary  convo- 
luted tubule,  which,  having  run  for  a  variable  distance,  becomes 
narrow  near  the  base  of  the  pyramid,  and  taking  a  comparatively 
straight  course  downward  enters  the  pyramid  under  the  name  of 
the  (3)  descending  limb  of  Henle's  loop;  some  of  these  run  nearly 
as  far  as  the  apex,  but  most  of  them  near  the  base  or  middle  of  the 
pyramid  turn  upward  forming  thus  (4)  Henle's  loop  and  be- 
ginning the  (5)  ascending  limb  of  Henle's  loop;  the  tube  having 
reentered  the  cortical  substance  becomes  convoluted  again, 
(6)  secondary  convolution,  which,  by  a  less  tortuous  continua- 
tion, the  (7)  intermediate  tube,  communicates  with  the  collecting 
tubules,  or  the  (8)  straight  tubes  of  Bellini;  these  last  beginning 
in  the  cortex,  and  receiving  in  their  course  large  numbers  of  inter- 
mediate tubes,  enter  the  base  of  the  pyramid  and  run  in  a  nearly 
straight  direction  toward  the  apex.  About  100  of  these  straight 
tubes  entering  at  the  base  join  in  their  course  downward  until 
at  the  apex  they  are  represented  by  a  single  tube.  These  collec- 
tions constitute  the  pyramids  of  Ferrein;  there  are  about  12-18 
pyramids  of  Ferrein  to  each  Malpighian  pyramid,  and  as  many 
tubal  orifices  at  the  apex.  The  so-called  zigzag  and  spiral 
tubules  are  here  considered  parts  of  the  first  and  second  convo- 
luted tubules.  (See  Fig.  58.) 

Before  they  reach  the  collecting  tubules  the  tubes  vary  in  diam- 
eter from  ToVo  to  ToVfr  inch;  the  collecting  tubules  progressively 
increase  in  diameter  from  -^  to  -%fa  inch.  The  cells  lining  the 
convoluted  and  intermediate  tubules  are  inclined  to  the  pyramidal 


198  EXCRETION   BY   THE   KIDNEYS  AND   SKIN 


FIG.  58. — A  diagram  of  the  sections  of  uriniferous  tubes. 
A,  cortex  limited  externally  by  the  capsule;  a,  subcapsular  layer  not  containing 
Malpighian  corpuscles;  a',  inner  stratum  of  cortex,  also  without  Malpighian  capsules; 
B,  boundary  layer;  C,  medullary  part  next  the  boundary  layer;  i,  Bowman's  cap- 
sule of  Malpighian  corpuscle;  2,  neck  of  capsule;  3,  first  convoluted  tubule;  4,  spiral 
tubule;  5,  descending  limb  of  Henle's  loop;  6,  the  loop  proper;  7,  thick  part  of  the 
ascending  limb;  8,  spiral  part  of  ascending  limb;  9,  narrow  ascending  limb  in  the 
medullary  ray;  10,  the  zigzag  tubule;  n,  the  second  convoluted  tubule;  12,  the 
functional  tubule:  13,  the  collecting  tubule  of  the  medullary  ray;  14,  the  collecting 
tube  of  the  boundary  layer;  15,  duct  of  Bellini.  (Kirkes  after  Klein.) 


STRUCTURE    OF    THE   KIDNEY  199 

shape.  Their  bases  present  the  appearance  of  fibers  at  right 
angles  to  the  basement  membrane  (hence  "rodded"  cells),  while 
their  opposite  extremities  are  granular.  The  tubes  of  Henle 
are  lined  by  flattened  epithelium  for  the  most  part. 

The  division  is  somewhat  arbitrary,  but  the  secreting  portion 
of  the  tubules  is  supposed  to  be  confined  to  the  cortical  substance, 
while  the  tubes  of  the  medullary  substance  only  carry  away  the 
fluid. 

Blood  Supply. — The  renal  artery,  having  entered  the  hilum, 
divides  into  branches,  two  of  which  usually  enter  each  column 
of  Bertin.  Running  upward  in  these  columns  the  branches  give 
off  small  arterial  twigs  to  the  substance  of  the  column.  When 
a  point  opposite  the  bases  of  the  Malpighian  pyramids  is  reached 
each  branch  follows  the  convex  base  of  the  pyramid  to  which  it 
is  adjacent,  the  one  branch  going  in  an  opposite  direction  to 
the  other.  Each  meets  a  corresponding  branch  from  the  other 
side  of  the  pyramid,  and  thus  a  convex  arterial  arch  covers  the 
base  of  the  pyramid,  from  which  arch  branches  go  inward  to 
supply  the  medullary  substance  and  outward  to  furnish  branches 
to  the  glomeruli.  The  arrangement  of  the  vessels  in  relation  to 
the  Malpighian  bodies  has  been  noticed.  In  the  glomerulus  the 
capillaries  do  not  form  a  true  anastomosis,  but  this  is  not  true  of 
the  network  surrounding  the  convoluted  tubes. 

Mechanism  of  Urinary  Secretion. — Histologists  have  been 
unable  to  demonstrate  the  presence  of  distinct  secretory  fibers 
for  the  glomerular  or  tubal  cells.  This  leaves  the  mechanism  of 
secretion  to  be  explained  by  (i)  the  vascular  supply  and  by  (2) 
the  "vital  activity"  of  the  cells — both  operating  in  conjunction 
with  osmosis. 

Irritation  of  a  certain  part  of  the  floor  of  the  fourth  ventricle 
occasions  certain  marked  changes  in  the  quantity  and  quality  of 
the  urine;  secretion  of  the  upper  dorsal  cord  temporarily  arrests 
the  secretion;  mental  emotions,  such  as  fright,  anxiety,  etc.,  also 
modify  the  flow.  All  these  circumstances,  and  many  others, 


2OO 


EXCRETION   BY   THE    KIDNEYS   AND    SKIN 


indicate  some  control  over  the  activity  of  the  kidneys  by  the 
nervous  system;  but  that  influence  is  probably  exerted  only 
through  vase-constrictor  and  vaso-dilator  fibers  to  the  vessels. 
Assuming  for  the  present  that  nearly  all  the  constituents  of 


FIG.  59. — Blood-vessels  of  the  kidney. 

A,  capillaries  of  cortex;  B,  of  medulla;  a,  interlobular  artery;  i,  vas  afferens;  2, 
vas  efferens; *', e,  vasa  recta;  VV,  interlobular  vein;  S,  origin  of  a  stellate  vein;  i,  i, 
Bowman's  capsule  and  glomerules:  P,  apex  of  papilla;  C,  capsule  of  kidney;  e,  vasa 
recta  from  lowest  vas  efferens.  (Stirling.) 


STRUCTURE  OF  THE  KIDNEY  2OI 

urine  preexist  in  the  blood  and  are  simply  taken  out  of  the 
circulation  in  the  kidney,  it  may  be  stated  that,  for  the  most 
part,  the  water  and  salts  are  extracted  by  the  cells  of  the  Mal- 
pighian  bodies,  while  the  urea  and  related  nitrogenous  solids  are 
removed  by  the  cells  of  the  convoluted  tubes ;  so  that  the  specific 
gravity  of  the  fluid  is  raised  in  passing  down  the  tubes.  While 
the  histology  of  the  kidney,  and  especially  the  arrangement 
of  the  glomeruli,  is  most  favorable  for  the  exercise  of  simple 
osmosis,  and  while  this  process  is  doubtless  mainly  responsible 
for  the  phenomena  which  occur,  it  seems  highly  probable  that 
the  cells  themselves  modify  osmotic  action  by  taking  an  active 
part  in  the  secretion  of  urine.  They  undoubtedly  exercise  a 
sel&ctive  affinity  accounting  for  the  different  materials  handled 
by  the  glomeruli  and  the  tubes.  Moreover,  morphological 
changes  in  the  tubal  cells  during  activity  have  been  microscopic- 
ally demonstrated.  Vesicles  are  described  as  forming  in  the 
body  of  the  cell,  approaching  the  lumen,  bursting  and  discharg- 
ing their  contents — which  are  supposed  to  include  the  urea  and 
such  other  materials  as  may  be  here  extracted  from  the  blood. 

As  regards  the  elimination  of  water  and  salts  by  the  glomerular 
epithelium,  it  must  also  be  admitted  that  the  cells  take  some 
obscure  but  active  part.  Were  this  only  an  osmotic  process 
the  amount  eliminated  would  vary  exactly  as  the  pressure. 
While  usually  a  rise  in  renal  blood-pressure  is  accompanied 
by  an  increased  flow  of  urine  and  a  fall  by  a  correspondingly 
decreased  flow,  the  rule  does  not  always  hold  good.  For  in- 
stance, compression  of  the  renal  vein  raises  the  pressure  but 
does  not  increase  the  amount  of  urine. 

Another  fact,  which  seems  almost  if  not  quite  as  invariable  as 
the  effect  of  blood-pressure,  is  that  the  amount  of  urine  varies 
directly  as  the  amount  of  blood  passing  through  the  kidney,  inde- 
pendently of  the  pressure;  and  these  two  facts  constitute  about 
all  that  is  definitely  known  concerning  the  local  conditions 
affecting  the  amount  of  urine.  Whether  diuretics  increase  the 


202  EXCRETION   BY   THE   KIDNEYS  AND    SKIN 

urinary  flow  by  simply  drawing  water  from  the  tissues  into  the 
blood  and  thus  increasing  the  amount  and  pressure,  or  by 
stimulating  the  cells  of  the  glomeruli  to  increased  functional 
activity  is  a  matter  as  yet  undetermined. 

Properties  and  Composition  of  Urine. — When  an  ordinary 
amount  of  liquid  is  ingested  and  when  the  skin  is  moderately 
active  the  urine,  in  normal  conditions,  has  a  clear  reddish  amber 
color  and  a  specific  gravity  of  about  1020.  The  more  fluid 
ingested  the  paler  will  be  the  color  and  the  lower  the  specific 
gravity;  the  more  active  the  skin  the  higher  will  be  the  color  and 
specific  gravity.  The  urine  is  diluted  in  the  first  case  and  con- 
centrated in  the  second.  The  fact  is,  the  amount  of  solids  (rep- 
resented by  urea)  to  be  eliminated  in  24  hours  remains  approx- 
imately the  same,  and  those  solids  will  cause  a  high  or  low 
specific  gravity  according  as  little  or  much  water  is  eliminated 
with  them.  The  average  amount  of  urine  for  a  day  is  2  or  3 
pints.  Normally  it  has  an  acid  reaction  from  the  presence,  not  of 
a  free  acid,  but  of  acid  salts — chiefly  acid. sodium  phosphate. 
The  odor  is  not  disagreeable  on  ejection,  but  decomposition 
soon  begins  and  a  characteristic  offensive,  ammoniacal  odor 
develops. 

The  kidney  is  the  most  important  excretory  organ  in  the  body 
and  the  large  number  of  urinary  constituents  is  not  surprising. 
The  chief  organic  constituents  are  urea,  uric  acid,  hippuric  acid, 
xanthin,  hypoxanthin,  creatinin,  phenol,  indican,  oxalic  acid, 
lactates,  etc.  The  phosphates,  nitrates,  sodium  chloride,  and  car- 
bon dioxide  are  the  chief  inorganic  materials. 

Urea  is  the  most  important  of  the  nitrogenous  constituents. 
It  contains  a  large  amount  of  nitrogen.  Nearly  all  of  it  is 
removed  from  the  body  by  the  kidneys,  and  double  nephrectomy 
means  death  from  its  retention.  Its  formation  is  constant  and 
its  removal  necessary.  Its  presence  in  the  blood  seems  to  be 
the  normal  stimulus  exciting  the  activity  of  the  cells  of  the  con- 
voluted tubes. 


FORMATION   OF   UREA  203 

Whether  urea  is  produced  directly  in  the  tissues,  or  whether 
only  certain  substances  antecedent  to  it  are  there  formed,  it 
cannot  be  doubted  that  it  is  the  chief  final  product  of  nitrogen- 
ous ingesta  and  nitrogenous  dissimilation.  It  is  practically 
the  only  way  in  which  the  nitrogen  of  proteid  foods  can  escape 
from  the  body.  It  exists  not  only  in  the  blood  but  in  the  lymph, 
vitreous  humor,  sweat,  milk,  saliva,  etc.  It  has  been  stated  that 
the  taking  of  large  quantities  of  liquids  lowers  the  specific  gravity 
of  the  urine  by  diluting  it;  this  is  true,  but  the  actual  amount  of 
urea  is  increased  somewhat  by  such  a  procedure.  It  is  not  sur- 
prising that  the  quantity  of  urea  is  largely  increased  when  much 
nitrogenous  food  is  taken,  and  that  it  is  greatly  decreased  by  an 
exclusively  vegetable  diet.  Anything,  like  exercise,  which  will 
increase  actual  tissue  metabolism,  will  increase  the  output  of  urea, 
while  anything  retarding  tissue  metabolism,  like  alcohol,  will 
decrease  the  output.  The  average  amount  of  urea  for  24  hours 
is  350  to  450  grains. 

Formation  of  Urea. — Seeing  that  urea  is  the  typical  end  product 
of  the  physiological  oxidation  of  the  proteids,  it  becomes  of 
interest  to  determine,  if  possible,  where  urea  formation  takes 
place.  It  is  known  that  the  liver  is  very  active  in  producing 
this  substance;  but  it  is  not  alone  by  this  organ  that  urea  is 
formed.  At  the  present  time  the  prevailing  opinion  is  that,  for 
the  most  part,  the  proteids  under  destructive  metabolism  in  the 
tissues  do  not  reach  the  urea  stage  of  transformation,  but  are 
converted  into  ammonia  compounds  (which  differ  very  slightly 
from  the  urea  in  chemical  composition),  and  these  compounds 
are  conveyed  by  the  blood  to  the  liver,  where  the  slight  change 
necessary  to  make  them  urea  is  effected  under  the  influence  of 
this  organ.  Ammonium  carbamate  seems  the  typical  compound, 
but  ammonium  carbonate  and  others  are  probably  likewise  con- 
verted. Artificial  circulation  of  these  compounds  through  the 
liver  gives  rise  to  urea;  removal  of  the  liver  increases  the  am- 
monia compounds  and  decreases  the  urea  in  the  urine;  ammonia 


204  EXCRETION   BY   THE   KIDNEYS  AND    SKIN 

compounds  are  normally  very  much  more  abundant  in  the  portal 
blood  than  in  the  arterial,  but  when  the  liver  is  removed  they 
are  evenly  distributed  throughout  the  circulation,  and  the  animal 
dies  in  a  few  days  of  symptoms  which  can  be  aggravated  by  ad- 
ministration of  the  ammonia  compounds; — all  of  which  circum- 
stances go  to  show  that  it  is  ammonia  compounds  which  the  tissues 
produce,  and  that  they  are  changed  to  urea  in  the  liver. 

Still,  removal  of  the  liver  does  not  suspend  entirely  the  out- 
put of  urea.  Consequently  this  substance  must  be  formed  else- 
where, but  by  what  organs  is  unknown.  It  is  not  impossible 
that  it  is  formed  to  some  extent  in  all  organs  where  proteid  dis- 
sociation is  progressing.  This  is  practically,  if  not  really,  the 
case  in  health  at  any  rate,  even  under  the  theory  above  mentioned. 

It  is  to  be  noted  that  urea  is  not  full  oxidized;  it  can  be  oxidized 
outside  the  body.  Thus  the  heat-producing  capacity  of  the  pro- 
teids  is  not  completely  utilized.  If  they  have  been  broken  down 
in  the  body  into  substances  simpler  than  urea,  then  the  amount 
of  heat  liberated  in  such  dissociation  is  consumed  in  building  up 
the  urea  molecule  to  be  discharged. 

Uric  acid  is  combined  in  normal  urine  to  form  the  urates  of 
sodium,  potassium,  magnesium,  calcium  and  ammonium.  The 
urate  of  sodium  is  by  far  the  most  abundant  of  these,  and,  be- 
sides urate  of  potassium,  only  traces  of  the  others  are  found. 
Free  uric  acid  in  human  urine  is  pathological.  The  urates,  like 
urea,  come  ultimately  from  oxidation  of  the  nitrogenous  con- 
stituents of  the  body.  They  are  not  formed  in  the  kidney,  but 
pass  out  as  such  from  the  blood.  About  9-14  gr.  are  discharged 
daily.  The  amount  is  increased  in  gout. 

In  some  animals  uric  acid  takes  the  place  of  urea,  none  of  the 
latter  being  formed.  In  these  cases  it  is  manufactured  by  the 
liver  from  ammonia  compounds.  This  does  not,  however,  seem 
to  be  the  origin  of  uric  acid  in  human  urine.  It  has  been  looked 
upon  as  unconverted  urea,  i.  e.,  as  a  product  antecedent  to  urea; 
but  at  present  such  does  not  seem  to  be  the  case.  A  theory  that  it 


HIPPURIC  ACID  205 

is  the  end  product  of  the  destruction  of  certain  materials  in  the 
nuclei  of  cells  has  considerable  support. 

Hippuric  acid  exists  in  the  urine  as  hippurates.  It  differs 
from  most  of  the  other  urinary  constituents  in  being  formed  in 
the  kidney;  it  does  not  preexist  in  the  blood.  The  daily  output 
of  this  substance  is  about  10  grains,  though  the  amount  may  be 
considerably  increased  on  a  vegetable  diet.  The  benzoic  acid 
of  vegetables  seems  to  be  synthesized  into  hippuric.  In  proteid 
dissimilation  some  benzoic  acid  may  be  produced  and  eliminated 
in  this  shape. 

The  various  lactates  are  not  formed  by  the  kidney,  but  pass 
unchanged  into  it  from  the  blood.  The  lactic  acid  from  which 
they  are  formed  probably  results  from  the  transformation  of 
dextrose. 

Creatinin  is  normally  present  in  the  urine.  It  is  a  nitrogenous 
body  differing  from  creatin  only  by  a  molecule  of  water.  It  is 
eliminated  to  the  extent  of  about  15  grains  per  day.  A  part 
comes  from  proteid  destruction  in  the  body,  and  another  part 
is  said  to  come  directly,  without  metabolism,  from  creatin  which 
is  a  constituent  of  ordinary  meat.  It  is  not  formed  in  the 
kidney. 

Xanthin,  hyp oxan thin,  etc.,  are  to  be  regarded  as  nitro- 
genous excreta  allied  to  uric  acid  and  resulting  in  some  way 
from  proteid  metabolism.  They  are  regarded  by  some  as  hav- 
ing the  same  probable  origin  as  uric  acid,  viz.,  the  disintegration 
of  cell  nuclei. 

The  non-nitrogenous  constituents  scarcely  deserve  separate 
mention.  It  is  through  the  kidney  that  the  largest  variety 
of  these  materials  are  discharged.  Certain  of  these  are 
constant,  but  the  wide  variety  of  such  materials  taken  into  the 
alimentary  canal  accounts  for  the  same  wide  variety  in  the  urine. 
The  proportion  of  inert  substances  in  the  blood  is  approximately 
constant — kept  so  by  the  removal  of  any  excess  by  the  kidneys 
chiefly. 


206  EXCRETION   BY   THE    KIDNEYS  AND    SKIN 

Sodium  chloride  is  eliminated  thus  to  the  extent  of  about  151 
grains  daily.  The  sulphates  are  unimportant.  About  25  grains 
are  excreted  daily.  The  phosphates  are  more  important,  the 
acid  sodium  phosphate  being  mainly  responsible  for  the  acid  re- 
action of  the  urine.  Nitrogen  and  carbon  dioxide  are  the  chief 
gases  to  be  found.  The  color  of  urine  is  due  to  a  substance, 
urochrome,  which  is  probably  formed  from  hemoglobin.  Some 
mucus  from  the  bladder  is  also  in  the  urine. 

Variation  in  Amount  and  Composition  of  Urine. — "Its 
constitution  is  varying  with  every  different  condition  of  nutrition, 
with  exercise,  bodily  and  mental,  with  sleep,  age,  sex,  diet,  res- 
piratory activity,  the  quantity  of  cutaneous  exhalation,  and  in- 
deed with  every  condition  which  affects  any  part  of  the  system. 
There  is  no  fluid  in  the  body  that  presents  such  a  variety  of  con- 
stituents as  a  constant  condition,  but  in  which  the  proportion  of 
these  constituents  is  so  variable"  (Flint). 

Prolonged  bodily  exercise  will  increase  the  amount  of  urea, 
but  the  urine  is  generally  decreased  in  quantity  because  perspi- 
ration is  more  active.  The  young  child  discharges  relatively 
much  more  urea  and  urine  than  the  adult.  The  female  dis- 
charges relatively  more  urine,  but  less  urea,  than  the  male.  Di- 
gestion increases  the  urinary  flow.  Climate  and  season  act  chiefly 
though  increasing  or  diminishing  cutaneous  activity.  Emotions 
of  various  kinds  may  give  rise  to  an  abundant  flow  of  pale  urine. 

Discharge  of  Urine. — On  leaving  the  pelvis  of  the  kidney  the 
urine  enters  the  ureters  and  passes  through  them  to  the  bladder, 
whence  it  is  discharged  per  urethram. 

The  ureters  run,  one  from  each  kidney,  downward  and  slightly 
inward  behind  the  peritoneum,  a  distance  of  some  18  inches  to 
the  base  of  the  bladder.  In  the  female  the  cervix  uteri  lies  be- 
tween the  two  ureters  just  before  they  enter  the  bladder.  They 
penetrate  the  bladder  wall  obliquely,  their  course  therein  being 
nearly  an  inch  long.  The  effect  of  this  arrangement  is  that  dis- 
tention  of  the  bladder  closes  the  opening  more  closely  instead 


MICTURITION  207 

of  causing  regurgitation  into  the  ureter.  The  ureter  is  com- 
posed of  three  coats.  The  outer  is  fibrous,  the  middle  muscular, 
and  the  internal  mucous. 

The  bladder  serves  as  a  reservoir  for  the  urine  until  such  time 
as  it  is  convenient  for  it  to  be  evacuated.  This  organ,  when 
empty,  lies  deep  in  the  pelvis  in  front  of  the  rectum  in  the  male 
and  of  the  uterus  in  the  female.  When  moderately  distended 
it  will  hold  about  a  pint,  has  an  ovoid  shape  and  rises  to  the 
brim  of  the  pelvis.  It  also  has  three  coats.  The  outer  is  perit- 
oneal, and  covers  the  posterior  and  small  parts  of  the  lateral 
and  anterior  surfaces  only.  Its  lower  limit  posteriorly  is  the 
entrance  of  the  ureters.  The  middle  layer  is  muscular.  The 
fibers,  which  are  non-striped,  are  disposed  in  three  sheets.  Their 
contraction  compresses  the  contents  from  all  directions.  Em- 
bracing the  neck  (outlet)  of  the  bladder  is  a  thick  band  of  plain 
muscle  tissue  known  as  the  sphincter  vesicce.  The  tonic  contrac- 
tion of  this  muscle  prevents  the  continual  escape  of  urine.  The 
inner  coat  of  the  bladder  is  mucous.  It  is  rather  thick,  and 
loosely  adherent  to  the  subjacent  muscular  coat  except  over  the 
corpus  trigonum  where  it  is  closely  attached.  The  corpus  trigo- 
num  is  a  triangular  body  of  fibrous  tissue  just  underneath  the 
mucous  membrane;  its  apex  is  at  the  origin  of  the  urethra,  and 
its  other  angles  are  at  the  vesical  openings  of  the  ureters. 

Absorption  from  the  intact  mucous  membrane  of  the  bladder 
takes  place  very  sparingly,  if  at  all.  Abrasions  of  the  membrane 
from  any  cause  allow  absorption  to  occur;  and  this  fact  may  be 
made  use  of  to  locate  lesions  giving  rise  to  hematuria.  Iodide 
of  potassium  injected  into  the  bladder  can  be  detected  in  the 
saliva  if  the  bladder  is  the  source  of  the  blood. 

Micturition. — When  the  bladder  has  become  moderately  full 
the  desire  to  expel  its  contents  arises.  The  act  of  micturition 
involves  relaxation  of  the  sphincter  vesicce  and  contraction  of  the 
muscular  walls  of  thejbladder  aided  by  the  abdominal  muscles 
and  those  of  the  urethra.  A  slight  contraction  of  the  abdominal 


208  EXCRETION   BY   THE   KIDNEYS  AND    SKIN 

muscles  compresses  the  bladder;  after  a  short  interval  the 
sphincter  relaxes  and  allows  the  stream  to  pass  out  through  the 
urethra.  When  the  act  has  been  begun  contraction  of  the  blad- 
der will  suffice  to  nearly  empty  the  organ,  but  complete  evacu- 
ation is  finally  brought  about  by  a  series  of  convulsive  con- 
tractions on  the  part  of  the  muscles  of  the  abdomen. 

The  center  controlling  the  reflex  nervous  phenomena  of 
micturition  is  opposite  to  the  fourth  lumbar  vertebra  in  the 
spinal  cord. 

THE  SKIN. 

Functions. — The  functions  of  the  skin  from  a  physical  stand- 
point are  sufficiently  apparent.  It  furnishes  protection  to  the 
underlying  parts,  preserves  the  general  contour  of  the  body, 
affords  lodgment  for  afferent  nerve  terminations,  and  thus  estab- 
lishes relations  between  ourselves  and  our  surroundings.  As  an 
organ  of  excretion  it  is  very  important,  and  in  fact  essential  to  life. 
While  various  materials,  such  as  urea  and  CO2,  are  thus  dis- 
charged from  the  body,  their  amount  is  more  or  less  inconse- 
quential, and  it  appears  that  it  is  the  action  of  the  skin  as  a 
regulator  of  heat  " excretion"  which  is  vital.  It  furnishes  one  of 
the  three  chief  routes  for  the  discharge  of  water  from  the  body, 
and  it  will  be  seen  that  it  is  largely  through  the  output  of  water 
that  the  output  of  heat  is  regulated.  So  necessary  is  the  skin  in 
this  respect  that  the  covering  with  impermeable  substances  of  as 
much  as  half  the  body  surface  is  followed  by  death. 

The  skin  excretions  are  contained  in  the  products  of  the  seba- 
ceous and  sweat  glands.  These  products  correspond  altogether 
to  neither  the  secretions  nor  the  excretions,  and  the  sebaceous 
glands  have  been  described  under  the  head  of  secretion.  It  is 
to  be  remembered,  however,  that  the  sweat  usually  represents 
part  of  the  sebaceous  as  well  as  the  sudoriparous  secretion, 
because  the  mixture  of  the  two  is  a  physical  necessity.  It  is  the 
water  of  the  sweat  which  is  the  most  important  excretion  from  the 


STRUCTURE    OF    THE    SKIN 


209 


skin,  although  the  elimination  of  CO2  and  inorganic  salts,  and 
especially  of  urea  in  some  pathological  conditions,  is  not  to  be 
overlooked. 

Structure. — The  skin  consists  of  an  external  covering,  the 
epidermis,  with  its  modifications,  hair  and  nails,  and  of  the  cutis 


Stratum  corneum. 

Stratum  lucidum. 
Stratum  granulosum. 


Stratum  Malpighii. 


FIG.  60. — Vertical  section  of  the  human  epidermis. 
The  nerve-fibrils,  n,  b,  stained  with  gold  chloride.     (Landois.) 

vera.     Imbedded  in  the  cutis  vera  are  sebaceous  and  sweat  glands 
and  hair-follicles.     (Fig  61.) 

Epidermis. — The  epidermis  consists  of  at  least  four  layers 
of  epithelial  cells.     From  above  downward  these  are  (i)  the 
stratum  corneum,  (2)  the  stratum  lucidum,  (3)  the  stratum  granu- 
14 


210  EXCRETION   BY   THE   KIDNEYS  AND    SKIN 

losum,  (4)  the  rete  mucosum  or  Malpighii.  All  these  except  the 
stratum  corneum  have  a  fairly  constant  thickness.  The  stratum 
corneum  is  thick  or  thin  according  to  location  and  degree  of 
exposure,  and  its  cells  are  flat  and  horny.  The  lowest  cells  of 
the  rete  mucosum  are  columnar.  From  this  last-named  layer 
the  cells  pass  gradually  upward,  and  as  gradually  assume  the 
shape  of  the  horny  layer.  The  horny  cells  are  thrown  off  and 
their  place  is  taken  by  others  from  beneath.  (Fig  60.) 

Hairs  are  to  be  found  on  almost  all  parts  of  the  cutaneous 
surface.  They  consist  of  a  bulb  and  a  shaft.  A  depression  of 
the  skin  involving  both  epidermis  and  cutis  vera  constitutes  the 
hair-follicle  in  which  the  bulb  rests.  A  projection  at  the  bottom 
of  the  follicle  corresponds  to  a  papilla,  and  upon  it  the  bulb  is 
placed.  The  shaft  has  an  oval  shape  in  cross  section.  It  is  com- 
posed of  fibrous  tissue,  outside  which  is  a  layer  of  imbricated 
cells. 

Nails  consist  of  a  superficial  layer  of  horny  cells  and  a  deeper 
one  corresponding  to  the  rete  mucosum.  The  root  of  the  nail 
is  received  into  the  matrix — a  specialized  portion  of  the  cutis 
vera. 

Cutis  Vera. — The  cutis  vera  is  tough  but  elastic.  It  rests 
upon  cellular  and  adipose  tissue.  Its  structure  is  areolar  with 
some  non-striated  muscle  fibers.  Projecting  from  the  cutis  vera 
into  the  epidermis  are  minute  conical  elevations,  the  papilla. 
Many  of  them  contain  sensory  nerve  terminals. 

Sweat  Glands. — Practically  the  whole  cutaneous  surface  con- 
tains sweat  glands.  Some  two  and  a  half  millions  are  thought  to 
exist  in  the  skin  of  the  average  individual.  They  are  particularly 
abundant  in  the  skin  of  the  palms  of  the  hands  and  soles  of  the 
feet.  They  belong  to  the  simple  tubular  type,  and  consist  of  a 
secreting  portion  and  an  excretory  duct.  The  secreting  part  lies 
just  underneath  the  true  skin  and,  as  a  whole,  resembles  a  small 
nodule;  however,  the  nodule  consists  of  an  intricate  coiling  of  the 
tube  itself  which  is  of  approximately  uniform  diameter  throughout. 


SWEAT   GLANDS 


211 


It  curls  upon  itself  some  6-12  times  and  ends  by  a  blind  extrem- 
ity.    It  is  lined  by  epithelial  cells. 


FIG.  61. — Vertical  section  of  skin. 

A,  sebaceous  gland  opening  into  hair-follicle;  B,  muscular  fibers;  C,  sudoriferous 
or  sweat-gland:  D,  subcutaneous  fat;  E,  fundus  of  hair-follicle,  with  hair-papilla. 
(Kirkes  after  Klein.) 

The  duct  passes  away  from  the  glandular  coil,  runs  through 
the  cutis  vera  in  a  comparatively  straight  course  and  assumes  a 


212  EXCRETION   BY   THE   KIDNEYS  AND    SKIN 

spiral  shape  as  it  traverses  the  epidermis  to  open  obliquely  on 
the  surface.  With  the  ducts  of  the  larger  glands  are  connected 
a  few  non-striped  muscular  fibers  which  may  aid  in  the  discharge 
of  the  secretion.  (Fig  61.) 

Properties  and  Composition  of  Sweat. — The  secretion  is 
colorless,  has  a  slight  characteristic  odor,  and  a  salty  taste.  Its 
specific  gravity  is  about  1003-4,  and  its  reaction  is  usually  acid 
when  just  discharged.  It  contains  a  large  proportion  of  water, 
a  little  urea  and  fatty  matter,  and  quite  a  quantity  of  inorganic 
salts  of  which  the  chief  is  sodium  chloride.  All  the  constituents 
in  health  are  of  subsidiary  importance  except  the  water.  Under 
average  conditions  of  temperature  and  exercise  the  amount 
secreted  in  24  hours  is  about  2  pounds.  But  the  quantity  is 
very  variable — as  much  so  as  the  urine,  and  may  be  said  in  a 
general  way  to  vary  inversely  as  the  urinary  secretion. 

Mechanism  of  the  Secretion  of  Sweat.— Sweat  is  produced 
continuously,  though  up  to  a  certain  point  it  passes  off  as  vapor 
or  "insensible  perspiration."  Beyond  that  point  it  accumulates 
on  the  skin  as  an  evident  fluid  and  becomes  "  sensible  perspira- 
tion." Whether  it  escapes  as  sensible  or  insensible  perspiration, 
it  is  secreted  as  a, fluid. 

The  activity  of  the  cells  lining  the  glandular  coils  in  separat- 
ing sweat  from  the  blood  is  undoubted.  Distinct  secretory  fibers 
are  distributed  to  them,  and  through  the  influence  of  these  fibers 
the  glands  will  secrete  sweat  even  without  an  increase  in  the 
blood  supply.  But  usually  a  determination  of  blood  to  the  sur- 
face means  an  increase  of  perspiration.  This  occurs  during 
violent  exercise,  e.  g.  However,  that  the  production  of  sweat  is 
not  altogether  dependent  on  this  factor  is  shown  by  profound 
sweating  in  shock,  nausea  and  like  conditions  when  the  skin  is 
pale  and  cold,  and  by  dryness  of  the  flushed  skin  in  febrile 
diseases.  Furthermore,  experiments  on  inferior  animals  have 
revealed  fibers  which  influence  the  secretion  of  sweat  without 
affecting  the  blood  flow. 


SECRETION   OF    SWEAT  213 

Practically,  in  health,  the  only  conditions  which  increase  the 
flow  of  perspiration  are  muscular  exercise  and  a  high  external 
temperature.  Of  these,  exercise  probably  works  through  the 
nerve  centers;  external  heat  does  not  stimulate  the  glands  directly, 
but  irritates  the  cutaneous  terminations  of  afferent  fibers  which 
convey  impressions  to  the  sweat  centers,  whence  messages  are 
sent  out  by  secretory  fibers  to  the  glandular  epithelium  and 
their  activity  begins.  In  both  cases  there  is  accompanying 
dilatation  of  the  superficial  vessels  under  the  influence  of  the 
vaso-dilator  fibers. 

It  is  supposed  that  the  chief  center  is  in  the  medulla  oblon- 
gata  and  that  secondary  centers  exist  in  the  lumbar  region  of 
the  cord. 

The  amount  of  CO2  eliminated  by  the  skin  is  inconsiderable 
in  the  human  being. 


CHAPTER  XL 
THE  NERVOUS  SYSTEM. 

General  Functions  of  the  System  as  a  Whole. — The  nervous 
system  is  the  most  delicately  organized  part  of  the  animal  body. 
Its  sensory  terminations  receive  impressions  which  are  conducted 
to  the  centers;  it  conveys  impulses  from  the  centers  to  the  different 
parts  of  the  body,  controlling  and  regulating  their  action.  Con- 
necting, as  it  does,  all  parts  of  the  organism  into  a  coordinate 
whole,  it  is  the  .only  medium  through  which  impressions  are  re- 
ceived, and  is  the  only  agency  through  which  are  regulated  move- 
ment, secretion,  calorification  and  all  the  processes  of  organic  life. 
This  system,  ramified  throughout  the  body,  connected  with  and 
passing  between  its  various  organs,  serves  them  as  a  bond  of  union 
with  each  other,  as  well  as  with  the  brain.  The  mind  influences 
the  corporeal  organs  through  the  instrumentality  of  this  system, 
as  when  volition  calls  them  into  action;  on  the  other  hand, 
•changes  in  the  organs  of  the  body  may  affect  the  mind  through 
the  same  channel,  as  when,  for  instance,  pain  is  mentally  per- 
ceived when  the  finger  is  burned.  Thus  it  is  that  the  nervous 
system  becomes  the  main  agent  in  what  is  known  as  the  "life 
of  relation;"  for  without  some  medium  for  the  transmission  of 
its  mandates,  or  some  means  of  receiving  those  impressions  which 
external  objects  are  capable  of  exciting,  the  mind  would  be 
completely  isolated,  and  could  hold  no  communion  with  the 
external  world. 

It  should  not  be  understood,  however,  that  the  nervous  system 
cannot  operate  independently  of  mental  influence.  All  those 
manifestations  of  nervous  activity  connected  with  the  perform- 

214 


GENERAL    FUNCTIONS  215 

ance  of  the  so-called  "organic  functions"  of  life  as  digestion, 
circulation,  etc.,  are  not  directly  influenced  by  volition;  indeed 
an  essential  character  of  these  functions  is  that  they  are  com- 
pletely removed  from  the  influence  of  the  will;  to  be  conscious 
subjectively  of  their  performance  is  an  evidence  of  abnormality. 

The  first  step  in  evefy  voluntary  act  is  a  mental  change,  in 
which  the  act  of  volition  consists.  If  this  mental  change  be  of 
such  nature  as  to  direct  its  influence  upon  a  muscle,  or  a  particu- 
lar set  of  muscles,  the  contraction  of  those  muscles  immediately 
supervenes,  so  as  to  bring  about  the  predetermined  voluntary  act. 
But  the  influence  of  the  will  could  not  possibly  be  exerted  upon 
those  muscles  except  through  intervention  of  the  nerves. 

Furthermore,  a  certain  mental  state,  in  cases  of  common  or 
special  sensation,  is  induced  by  an  impression  made  upon  certain 
bodily  organs.  But  in  no  case  could  the  mental  state  be  pro- 
duced unless  a  particular  part  of  the  nervous  system  were  present 
to  convey  the  impression  received  to  the  center  capable  of  rec- 
ognizing it.  If  the  hand  be  burned  pain  is  felt,  but  were  the 
nerves  not  present  to  convey  the  impression  made  by  the  heat 
no  degree  of  temperature  could  make  the  mind  cognizant  of 
injury.  When  light  is  admitted  to  the  eye  a  corresponding 
mental  sensation  is  produced,  but  for  the  production  of  this  the 
integrity  of  the  optic  nerve  is  a  necessary  condition. 

It  will  be  gathered  from  the  foregoing  remarks  that  the  nervous 
system  is  not  only  capable  of  conveying  communications,  but  that 
it  has  the  power,  in  certain  of  its  divisions,  of  receiving  im- 
pressions and  of  giving  rise  to  stimulating  influences — that  is, 
that  it  is  capable  of  generating  a  peculiar  power  known  as  "nerve 
force"  It  thus  becomes  the  seat  of  distribution  of  energy  to  all 
the  cells.  These  generating  parts  of  the  system  are  the  reservoirs 
of  force — force  which  has  been  derived  from  the  cells  and  is  dis- 
tributed to  them.  This  nervous  force,  having  its  origin  in  the 
living  activity  of  the  cells,  is  the  highest  manifestation  of  vital 
energy. 


2l6  THE   NERVOUS    SYSTEM 

The  nervous  structure  is  divided  into  two  great  systems: 

1.  The  Cerebro-spinal  System  consists  of  the   brain,  the 
spinal  cord  and  all  the  nerves  which  run  off  from  these.     This 
system  is  especially  concerned  with  the  functions  of  relation,  or 
of  animal  life.     It  presides  over  general  and  special  sensation, 
over  voluntary  movements,  over  intellection,  over  all  conscious 
activity,  and  over  all  other  functions  which  are  peculiar  to  the 
animal.     It  is  by  this  system  that  we  know  of  and  deal  with 
the  other  great  system. 

2.  The  Sympathetic,  or  Ganglionic  System  is  especially 
connected  with   the  functions   relating   to  nutrition — functions 
similar  to  those  occurring  in  the  vegetable  kingdom.     It  pre- 
sides over  all  organic  life — over  all  unconscious  activity.    While 
the  operations  over  which   this  system   holds  sway  are   quite 
different   from   those    under   the   supervision  of    the   cerebro- 
spinal  system,   it  must  not   be   concluded   that   the  two  are 
not  anatomically  and  physiologically  related.     Neither  is  inde- 
pendent of  the  other,  as  was  once  thought,  but  both  are  parts 
of  the  same  great  apparatus. 

Divisions  of  the  Nervous  Substance  as  a  Whole. — The 
nervous  matter,  irrespective  of  the  two  systems,  may  be  studied 
as  consisting  of  two  divisions.  The  first  is  made  up  of  cells ; 
the  second  of  tubes,  or  fibers.  Although  the  tissue  may  be  thus 
divided  into  nerve  cells  and  nerve  fibers,  the  present  conception 
of  the  arrangement  of  the  nervous  substance  is  that  these  two 
are  only  different  parts  of  the  same  element  known  as  the 
neuron,  supported  by  tissue  elements  known  as  neuroglia,  which, 
though  not  identical  with  connective  tissue,  is  comparable  to  it 
in  its  function  of  support.  The  neuron,  thus  considered,  consists 
of  a  protoplasmic  body  which  sends  out  a  number  of  branching 
processes  called  dendrites,  one  of  which  becomes  the  axis  cylinder. 
While,  therefore  it  is  to  be  understood  that  the  cell  and  the  fiber 
in  the  nervous  system  are  both  portions  of  an  identical  whole,  a 


NERVE    FIBERS  2 17 

description  of  them  as  separate  parts  is  warranted  for  the  sake  of 
convenience  and  by  differences  in  their  general  characteristics. 

The  nerve  cells  are  the  only  organs  capable,  under  any  circum- 
stances, of  generating  nerve  force.  As  a  rule  they  are  stimu- 
lated to  generate  this  force  by  the  reception  of  an  impression 
through  the  nerve  fiber,  but  they  may  in  some  cases  be  directly 
excited  by  mechanical,  electrical  or  chemical  means.  They  also 
frequently  act  as  conductors,  as  will  be  seen  later. 

Under  no  circumstances  can  nerve  fibers  generate  force  Their 
office  is  exclusively  to  conduct  impressions  and  impulses,  and 
they  usually  receive  these  impressions  and  impulses  at  their 
terminal  extremities  in  the  case  of  afferent  nerves,  and  from  the 
centers  in  the  case  of  efferent  nerves;  but  in  many  instances 
they  may  be  stimulated  in  any  part  of  their  course.  Some  fibers 
are  incapable  of  being  thus  directly  stimulated.  The  nerves  of 
special  sense  are  insensible  to  direct  stimulation. 

Nerve  Fibers. — Nerve  fibers  are  of  two  kinds:  (A)  white  or 
medullated  fibers  and  (B)  gray  or  non-medullated  fibers.  The 
non-medullated  fibers  possess  the  conducting  elements  alone, 
while  the  medullated  possess  certain  accessory  anatomical 
elements. 

(A)  Each  medullated  fiber  has  (i)  an  external  enveloping 
membrane  called  the  neurilemma,  or  the  primitive  nerve  sheath, 
or  the  sheath  of  Schwann;  (2)  an  intermediate  substance  known 
as  the  myeline  sheath,  or  the  white  substance  of  Schwann,  or  the 
medullary  substance;  (3)  a  central  fiber,  the  true  conducting 
element,  which  usually  goes  under  the  name  of  the  axis  cylinder, 
or  axone. 

The  sheath  of  Schwann  is  analogous  to  the  sarcolemma  of 
muscle  fibers.  It  is  a  structureless  protective  membrane,  some- 
what elastic,  and  presents  oval  nuclei  with  their  long  diameter 
corresponding  to  the  direction  of  the  fiber .  This  sheath  is  want- 
ing over  the  medullated  fibers  in  the  white  substance  of  the  brain 
and  spinal  cord. 


2l8 


THE   NERVOUS    SYSTEM 


•  Node  of  Ranvier' 


Primitive  sheath. 


•  Nerve  corpuscles. 


Axis  cylinder. 


White  substance, 
of  Schwann. 


Node  of  Ranvier. 


FIG.  62. — Scheme  of  a 
medullated  nerve  fiber  of  a 
rabbit  acted  on  by  osmic 

acid. 

The  incisures  are  omitted. 
X  400.  (Landois.) 


It  is  the  white  substance  of  Schwann 
which  gives  to  the  nerve  its  peculiar 
whitish  appearance.  This  is  a  fatty 
substance  of  a  semi-fluid  consistence. 
It  fills  the  tube  made  by  the  sheath  of 
Schwann  and  surrounds  the  axis  cylin- 
der. It  is  wanting  at  the  origin  of  the 
fibers  in  the  centers  and  at  their  periph- 
eral distribution.  It  is  probably  not 
necessary  to  conductivity.  In  fresh 
nerves  this  substance  is  strongly  refrac- 
tive, and  the  optical  effect  produced  by 
its  varying  thickness  in  the  center  and 
at  the  edges  is  the  appearance  of  dark 
borders.  It  easily  coagulates  into  an 
opaque  mass.  The  idea  that  the  mye- 
line  sheath  acts  as  an  insulator  lacks 
supporting  evidence.  The  theory  that 
it  is  nutritional  is  plausible;  but  no  suf- 
ficient difference  in  the  medullated  and 
non-medullated  fibers  in  this  respect 
has  been  found  to  establish  the  theory 
as  a  fact.  At  certain  points  in  the 
course  of  medullated  fibers  there  are 
seen  constrictions  called  the  nodes  of 
Ranvier.  At  these  points  the  medul- 
lary substance  is  wanting  and  the 
sheath  of  Schwann  is  in  contact  with 
the  axis  cyclinder.  It  is  not  improb- 
able that  these  nodes  furnish  a  mode 
of  access  for  the  nutrient  plasma. 
Certain  it  is  that  they  are  most  numer- 
ous where  physiological  activity  is 
supposed  to  be  most  active. 


NERVE    TRUNKS 


219 


The  axis  cylinder  is  composed  of  a  large  number  of  primitive 
fibrillae.  This  band  occupies  about  one- 
fourth  the  diameter  of  the  tube  and  is 
the  true  conducting  element,  as  is  shown 
by  its  invariable  presence,  its  continuity 
and  other  considerations  equally  con- 
clusive. It  is  demonstrated  under  the 
microscope  with  difficulty  in  fresh  speci- 
mens. It  is  directly  connected  with  a 
nerve  cell,  and  is  the  essential  part  of 
the  fiber.  The  process  of  the  cell  which 
becomes  the  axis  cylinder  is  not,  as  was 
once  thought  unbranched,  but  itself 
sends  off  "  collaterals "  in  the  gray  sub- 
stance. These  collaterals,  however,  do 
not  actually  join  any  other  nerve  cells 
or  fiber. 

The  average  diameter  of  medullated 
fiber  is  about  -pfc-y  in.,  though  all  are 
said  not  to  preserve  the  same  diameter 
throughout  their  course. 

(B)  The  non-medullated  fibers  (fib- 
ers of  Remak)  seem  to  be  simple  axis 
cylinders  without  the  other  atomical 
elements  peculiar  to  medullated  fibers. 
They  make  up  a  large  part  of  the  trunks 
and  branches  of  the  sympathetic  sys- 
tem, and  represent  the  filaments  of 
origin  and  distribution  of  all  nerves. 
They  are  thought  by  some  to  possess 
a  neurilemma.  They  are  pale  gray 
in  color. 

Nerve  Trunks. — The  above  remarks 
apply  to  a  single  nerve  fiber.  These 


FIG.  63. — Non-medullated 
nerve  fiber. 

Vagus  of  dog.  b,  fibrils;  u, 
nucleus;  p,  protoplasm  sur- 
rounding it.  (Stirling.) 

fibers  seldom  run  an 


220 


THE   NERVOUS    SYSTEM 


extended  course  alone,  but  are  bound  together  in  large  num- 
bers to  make  a  nerve  trunk.  This  trunk  is  composed  of  a 
number  of  bundles  of  fibers,  and  is  surrounded  by  a  con- 
nective tissue  membrane  known  as  the  epineurium;  the 
separate  bundles,  or  funiculi,  are  surrounded  each  by  a  similar 
membrane  called  the  perineurium;  while  inside  the  funiculi, 
between  the  primitive  fasciculi,  is  a  delicate  supporting  tissue 


FIG.  64. — Transverse  section  of  a  nerve.     (Median.) 
ep,  epineurium;  pe,  perineurium;  ed,  ehdoneurium.     (Landois.) 

known  as  the  endoneurium ,  or  the  sheath  of  Henle.  In  con- 
nection with  this  sheath  there  are  nuclei  belonging  to  the  con- 
nective tissue  and  to  the  nerve  fibers  themselves.  The  sheath  be- 
gins where  the  nerve  fibers  emerge  from  the  white  portion  of  the 
centers,  is  interrupted  by  the  ganglia  in  the  course  of  the  fibers, 
branches  as  the  bundle  branches,  and  is  lost  before  the  terminal 
distribution  is  reached.  It  is  seldom  found  surrounding  single 
fibers.  It  is  likewise  rare  for  capillaries  to  penetrate  it  and  reach 


NERVE    CELLS  221 

the  fibers  themselves.  There  are  numerous  lymph  spaces  around 
the  individual  fibers  as  well  as  around  the  funiculi.  In  situa- 
tions where  the  nerves  are  well  protected,  as  in  the  cranium,  the 
amount  of  fibrous  tissue  in  the  trunks  is  small,  but  where  oppo- 
site conditions  prevail,  as  in  muscular  substance,  this  tissue  is 
largely  increased  in  amount  as  regards  both  that  which  surrounds 
the  trunk  and  that  which  is  sent  m  between  the  funiculi  and 
fibers.  This  tissue  has  ramifying  in  it  a  network  of  fibers  known 
as  nervi  nervorum.  The  blood  supply  is  not  large. 

Individuality  of  Nerve  Fibers. — It  is  to  be  remembered  that 
so  far  as  can  be  'determined  every  nerve  fiber,  having  entered  a 
trunk,  proceeds  without  interruption  to  the  part  to  which  it  is 
finally  distributed,  whether  that  part  be  the  skin,  or  a  viscus,  or 
a  muscle,  or  a  gland,  or  some  organ  of  special  sense,  or  another 
nerve  cell,  or  what  not.  Collections  of  fibers  forming  bundles 
run  together  in  the  same  trunk,  may  leave  that  trunk  together, 
may  send  out  part  of  their  fibers  to  another  bundle  or  trunk,  or 
may  receive  other  fibers  from  other  funiculi;  but  everywhere  the 
relation  of  the  primitive  fibers  to  each  other  is  simply  one  of 
contiguity.  However,  as  the  axis  cylinder  approaches  the  seat 
of  its  final  distribution,  it  breaks  up  into  several  fibrillae,  such 
divisions  always  taking  place  at  the  nodes  of  Ranvier. 

Nerve  Centers. — The  nerve  centers  include  the  gray  matter 
of  the  brain  and  cord  and  the  ganglia  in  both  the  cerebro-spinal 
and  sympathetic  systems.  These  centers  have  a  gray  color  due 
to  the  presence  of  a  pigmentary  substance  in  the  cells  and  sur- 
rounding tissue.  The  ganglionic  centers  are  simple  collections 
of  nerve  cells  with  their  usual  accessory  elements — myelocytes, 
intercellular  granular  matter,  delicate  membranes  covering  some 
of  the  cells,  connective  tissue  elements,  blood-vessels  and 
lymphatics. 

Nerve  Cells. — These  are  irregular  in  shape  and  may  be  uni- 
polar, bipolar  or  multipolar.  They  also  vary  much  in  size.  The 
unipolar  cell  has  a  single  prolongation  which  becomes  the  axis 


222 


THE   NERVOUS    SYSTEM 


cylinder.  Bipolar  cells  are  prolonged  in  two  directions,  and  may 
be  looked  upon  as  simply  protoplasmic  enlargements  of  the  nerve 
fiber.  This  cell  is  frequently  covered  by  a  connective  tissue  en- 
velope which  is  continuous  in  both  directions  with  the  sheath  of 
Schwann.  Multipolar  cells  have  three  or  more  prolongations, 


Nerve  proc 
or  axone 


Neurilemma.  ......... 


—Neurilernma 


Nerve- cell. 


Terminal  . 
branches. 


A  B 

FIG.  65. 

A,  efferent  neuron;  B,  afferent  neuron. 


(Brubaker.) 


one  of  which  always  becomes  continuous  with  the  axis  cylinder 
and  is  called  the  axis-cylinder  process,  the  neuraxon,  or  the  axone. 
The  other  poles  branch  in  various  irregular  directions  like  the 
limbs  of  a  tree,  and  are  hence  called  dendrites.  They  also  go 
under  the  name  of  protoplasmic  prolongations.  Some  of  these 
unite  the  cells  to  contiguous  cells  by  interlacing  with,  but  not 


NEURONS  223 

actually  joining,  similar  poles  from  those  cells.  The  multipolar 
cells  in  the  anterior  cornua  of  gray  matter  of  the  cord  are  said 
to  be  larger  in  size  and  to  present  more  poles  than  corresponding 
cells  in  the  posterior  column. 

The  diameter  of  nerve  cells  varies  from  T2Vo  to  TCTO  *n>  ^he 
nucleus  is  usually  single,  and  most  cells  have  no  true  surrounding 
membrane.  If  a  nerve  fiber  be  followed  toward  the  center 
which  gives  it  origin  it  will  be  found  first  to  lose  its  sheath  and 
later  its  medullary  substance;  this  medullary  substance  may  con- 
tinue for  some  distance  after  the  sheath  is  lost,  as  in  the  white 
substance  of  the  encephalon,  but  never  penetrates  the  gray  sub- 
stance proper.  Every  nerve  fiber  is  connected  with  a  cell  by 
that  cell's  axis-cylinder  prolongation. 

Certain  retrograde  changes  take  place  in  the  neurons  in  old 
age — morphological  changes  agreeing  with  the  physiological 
decrease  in  energy-producing  power  at  that  time.  The  cell  body 
becomes  smaller,  the  dendrites  atrophy,  and  the  axones  diminish 
in  mass.  Nerve  "fatigue"  can  also  be  demonstrated  by  the 
microscope.  The  nuclei  of  the  sheath  are  flattened,  the  proto- 
plasm is  shrunken  and  vacuolated  and  the  nucleus  is  crenated. 
The  quantity  and  quality  of  the  food  may  be  perfect,  but  the 
power  of  the  cell  to  utilize  it  is  impaired,  and  this  means  dimin- 
ished physiological  power. 

Communication  Between  Different  Neurons. — Every  neu- 
ron is  anatomically  independent  of  every  other  neuron.  There  is  no 
actual  joining  of  fibers  or  dendrites — simply  an  interlacement 
of  the  end  arborizations.  This  is  illustrated  in  Figs.  62  and  63. 
In  the  latter  the  afferent  fiber  is  joined  to  no  cell  except  G,  one 
of  the  cells  of  the  spinal  root  ganglion.  Its  end  arborizations 
simply  interlace  with  the  dendrites  of  the  motor  cell  M.  C.  and 
cause  it  to  send  out  an  efferent  impulse  to  the  muscle  M. 

Furthermore,  there  are  frequent  relays  in  the  transmission  of 
nerve  messages.  By  no  means  do  all  the  fibers  from  the  motor 
area  of  the  brain  pass  themselves  out  as  parts  of  the  anterior  roots. 


224 


THE   NERVOUS    SYSTEM 


S.C. 


M.C: 


FIG.  66.— Reflex  action;  old  idea.     (Kirkes.) 


FIG.  67. — Reflex  action;  modern  idea.     (Kirkes.) 


NERVE   FIBERS 


The  relay  service  is  illustrated  in  Fig. 
64.  Here  again,  it  is  seen  that  there  is 
no  actual  joining  of  the  neurons.  When- 
ever it  is  said  that  a  nerve  cell  is 
" joined"  to  another,  or  that  the  axis 
cylinder  of  a  cell  "joins"  another  cell, 
no  actual  continuity  of  tissue  is  meant. 
Different  neurons  communicate  only  by 
contiguity. 

Peripheral  Nerve  Terminations. — 
Nerves  terminate  peripherally  (i)  in 
muscles,  (2)  in  glands,  (3)  in  special 
organs  connected  with  the  senses  of 
sight,  hearing,  smell  and  taste,  (4)  in 
hair-follicles,  (5)  in  simple  free  ex- 
tremities passing  between  epithelial  and 
other  cells,  and  (6)  in  several  kinds  of 
so-called  tactile  corpuscles. 

The  motor  nerves  passing  to  volun- 
tary muscles  form  first  a  "ground 
plexus"  for  each  group  of  muscle 
bundles — this  plexus  being  made  of 
the  axis-cylinder  fibrillae.  From  this 
plexus  fibrils  pass  to  form  an  "interme- 
diary plexus"  corresponding  to  each 
muscle  bundle.  These  fibrils  are  still 
medullated,  and  when  a  branch  from 
the  intermediary  plexus  enters  a  muscle 
fiber  its  sheath  becomes  continuous  with 
the  sarcolemma  of  that  fiber,  and  the 
axis-cylinder  fibrils  form  a  network  on 
the  surface  of  the  muscle  fiber.  This 
is  called  an  end  motorial  plate.  It  con- 
tains a  number  of  nuclei,  and  sends  off 
15 


[M 

FIG.  68. — Diagram  of  an 
element  of  the  motor  path. 

U.  S,  upper  segment;  L. 
S,  lower  segment ;  C.  C,  cell  of 
cerebral  cortex;  S.  C,  cell  of 
spinal  cord ,  in  anterior  cornu ; 
M,  the  muscle;  S,  path  from 
sensory  nerve  roots.  (Kirkes 
after  Cowers.) 


226  THE    NERVOUS    SYSTEM 

from  its  under  surface  fine  fibrillae  which  are  said  to  pass  between 
the  muscular  fibrillae  which  make  up  the  fiber.  Sensory  fibers 
are  somewhat  scantily  distributed  to  the  voluntary  muscles. 

In  plain  muscle  tissue  the  motor  nerves  are  distributed  after 
the  same  general  manner  as  in  the  striped  muscles,  though  with 
some  differences.  Here  the  fibers  are  not  medullated,  and 


Nerve-fibre. 


End-plate. 


Muscle  nucleus. 


FIG.  69. — Termination  of  a  nerve  fiber  in  end-plate  of  a  lizard's  muscle. 

(Stirling.) 

primitive  fibrils  passing  from  the  intermediary  plexus  finally 
enter  the  nuclei  of  the  muscle  cells. 

Medullated  fibers  have  been  traced  to  the  cells  of  glands, 
but  not  farther.  It  is  thought  by  some  that,  having  formed  a 
plexus,  non- medullated  fibeis  pass  in  to  terminate  in  the  nu- 
cleoli  of  the  gland  cells,  though  such  endings  have  not  been 
demonstrated. 

The  peripheral  distribution  of  nerves  connected  with  the 
special  senses  will  be  discussed  elsewhere. 

The  remaining  methods  of  termination  above  noted  apply  to 
afferent  nerves.  It  is  claimed  that  a  very  large  number  of  sen- 
sory nerves  terminate  in  hair-follicles.  If  such  be  the  case  it 
will  account  for  sensory  terminations  in  by  far  the  greater  part 
of  the  cutaneous  surface.  It  is  supposed  that  nerve  fibrillae  form 


NERVE    FIBERS 


227 


a  plexus  beneath  the  true  skin  and  send  branches  thence  to  the 
follicles,  though  the  exact  mode  of  termination  is  a  question  of 
some  obscurity. 

Terminations  between  epi- 
thelial cells  are  probably  more 
common  than  any  other  method 
of  sensory  distribution.  The 
fibers,  having  passed  to  the  sur- 
face of  the  skin  or  mucous  mem- 
brane, lose  everything  excepting 
the  axis  cylinder,  which,  dividing 
into  minute  ramifications,  passes, 
by  means  of  these  fibrillae,  among 
the  epithelial  cells.  This  mode 
of  termination  is  held  by  some 
to  prevail  in  the  glands.  It  cer- 
tainly prevails  in  parts  other 
than  the  skin  and  mucous  mem- 
branes. 

Sensory  nerves  further  termi- 
nate in  (a)  the  corpuscles  of 
P acini  or  Vater,  (b}  the  end  bulbs, 
or  tactile  corpuscles,  of  Krause, 
(c)  the  tactile  corpuscles  of 
Meissner,  (d)  the  tactile  menisques, 
and  (e)  the  corpuscles  ofGolgi. 

(a)  The  Pacinian  corpuscles 
are  oval  elongated  bodies.  Each 
corpuscular  body  has  a  length  of 
about  1*2-  of  an  inch,  and  is 
about  half  as  broad.  It  is  made 
up  of  a  number  of  concentric 
layers  of  connective  tissue  in  a  hyaline  ground  substance,  and  is 
attached  by  a  pedicle  to  the  nerve  whose  termination  it  is. 


FIG.  70. — Vater's  or  Pacini's 
corpuscle. 

a,  stalk;  b,  nerve  fiber  entering  it;  c, 
d,  connective-tissue  envelope;  e,  axis- 
cylinder  with  its  end  divided  at  /. 
(Landois.) 


228 


THE   NERVOUS    SYSTEM 


Through  this  pedicle  passes  a  single  (occasionally  more)  nerve 
fiber  which,  piercing  the  several  concentric  layers  constituting 
the  corpuscle,  gradually  loses  its  myeline  substance  and  runs 
longitudinally  through  the  center  of  the  body  to  terminate  at 
the  distal  end  of  the  central  cavity  in  a  knob-like  enlargement. 
These  corpuscles  are  found  in  great  abundance  on  the  palmar 
and  plantar  surfaces  of  the  hands  and  feet,  being  far  more 
numerous  on  the  first  phalanx  of  the  index  finger  than  elsewhere. 
About  six  hundred  are  said  to  be  present  in  each  hand  and  foot. 
They  are  also  to  be  found  on  the  dorsal  surfaces  of  the  hands 
and  feet,  over  parts  of  the  forearm,  arm  and  neck,  in  the  nipples, 
in  the  substance  of  muscles,  in  all  the  great  plexuses  of  the  sym- 
pathetic system,  and  in  numerous 
other  situations.  These  bodies  can- 
not be  considered  true  tactile  corpus- 
cles because  they  are  situated  beneath 
the  skin ;  neither  can  they  be  positively 
said  to  have  any  " special  sensory" 
function  such  as  the  appreciation  of 
temperature,  weight,  etc. 

(b)  The  end  bulbs  of  Krause  exist 
human  conjunctiva,  treated  in  great  number  in  the  conjunctiva, 
with  osmic  acid,  showing  the  glans  penis  and  clitoris,  the  lips, 
cells  of  core.  (From  Yeo  and  in  other  situations.  They  bear 
after  Longworth.) 

-.       ,        ,        ,     some   resemblance   to  the  corpuscles 

a,  nerve  fiber;   b,  nucleus  of 

sheath;  c,  nerve  fiber  within     of  Pacint,  but  are  much  less  elaborate 

core;  a,  cells  of  core. 

in    their    arrangement;    the    number 

of  concentric  layers  is  much  smaller,  while  the  contained 
mass  is  larger.  The  shape  is  spherical.  From  one  to  three 
medullated  fibers  pass  from  the  underlying  plexus  to  wind 
through  the  corpuscle  and  break  up  in  free  extremities.  The 
sheath  of  the  fiber  is  continuous  with  the  outer  covering  of  the 
corpuscle,  and  the  medulla  is  gradually  lost  as  the  fiber  enters 


FIG.  71. — End  bulb  from 


229 


NERVE   FIBERS 


the  bulb.     The  end  bulb  of  Krause  measures  from  yoV(r  to 
of  an  inch  in  diameter. 

(c)  The  tactile  corpuscles  of  Meissner  have  to  do  with  the 
sense  of  touch,  and  are  situated  largely  in  the  papillae  of  the 
skin  covering  the  palmar  surfaces  of  the  hands  and  the  plantar 
surfaces  of  the  feet ;  they  also  exist  in  other  situations,  correspond- 
ing in  general  to  the  distribution  of  the  Pacinian  corpuscles. 
The  largest  number  is  found  over  the  distal  phalanges  of  the 
fingers  and  toes  on  their  palmar  and  plantar  surfaces;  they 


FIG.  72. — Drawing  from  a  section  of  injected  skin. 

Showing  three  papillae,  the  central  one  containing  a  tactile  corpuscle,  a,  which  is 
connected  with  a  medullated  nerve,  and  those  at  each  side  are  occupied  by  vessels. 
(From  Yeo  after  Cadiat.) 

diminish  in  number  proximally  from  these  points.  They  may 
be  simple  or  compound  according  as  the  enclosing  capsule  con- 
tains one  or  more  collections  of  nucleated  cells.  Their  form  is 
oblong  with  the  long  axis  in  the  direction  of  the  papilla.  They 
vary  in  thickness  with  the  papillae  of  the  region  in  which  they 
are  located.  They  may  have  a  transverse  diameter  of  from 
"50  o  to  T4-^  of  an  inch,  and  probably  in  most  instances  occupy 
the  secondary  eminences  of  the  papillae  in  which  they  are  found. 
A  simple  papilla  does  not  generally  possess  both  vascular  and 
nervous  loops. 

(d)  The  tactile  menisques  are  found  in  certain  cutaneous 
regions.     Nerves  in  the  superficial  layer  of  the  skin  lose  their 


230  THE   NERVOUS    SYSTEM 

medullary  substance  and  divide  to  form  arborizations  which  are 
flattened  into  the  form  of  a  leaf. 

(e)  The  corpuscles  of  Golgi  are  situated  at  the  point  of  union 
of  tendons  with  muscles,  and  are  believed  by  some  to  have  to  do 
with  the  muscular  sense.  They  are  flattened  fusiform  bodies 
composed  of  granular  substance  enclosed  in  layers  of  hyaline 
membrane  and  containing  nervous  fibrillae. 

Properties  and  Classification  of  Nerve  Fibers. — Nerve 
fibers  are  for  the  purpose  of  conveying  messages  either  peripher- 
ally or  centrally.  They  may  be  stimulated  to  action  by  anything 
capable  of  suddenly  increasing  their  irritability.  In  any  case 
the  effect  of  the  stimulus,  whether  normal  or  abnormal,  is  mani- 
fested at  the  peripheral  distribution  of  the  stimulated  fiber.  So 
far  as  most  external  manifestations  are  concerned,  nerves  may 
be  classified  as  motor  and  sensory.  That  is  to  say,  stimulation, 
for  instance,  of  a  cerebro-spinal  nerve  (except  those  of  special 
sense)  is  followed,  under  ordinary  conditions,  by  one  of  two 
results — there  is  either  pain  or  contraction  of  a  muscle  to  which 
the  nerve  is  distributed.  This  is  a  typical  illustration  of  the 
action  of  motor  and  sensory  fibers,  and  the  manifestation  of 
nerve  action,  whether  it  consists  in  pain  or  motion,  is  a  result 
only  of  the  conduction  of  an  impression  of  an  impulse  to  the 
center  or  the  periphery.  It  is  to  be  noted  that  the  result  of  thus 
stimulating  a  nerve  fiber  is  manifested  at  one  extremity  only  of 
that  fiber,  and  always  at  the  same  extremity. 

However,  since  there  are  nerve  fibers  the  stimulation  of  which 
is  not  followed  by  pain  or  motion,  the  division  into  sensory  and 
motor  fibers  is  not  comprehensive  enough  to  include  all  the 
fibers  in  the  body.  But  since,  as  above  stated,  the  only  office  of 
fibers  is  to  conduct,  and  since  they  always  conduct  in  a  direction 
either  toward  or  away  from  the  center,  all  nerves  may  be  classified 
as  either  centripetal  or  centrifugal.  A  corresponding  division 
is  into  afferent  and  efferent.  It  will  be  seen  that  all  motor 
fibers  are  centrifugal  or  efferent,  but  not  all  centrifugal  or  efferent 


EFFERENT   NERVES  231 

fibers  are  motor.  It  will  likewise  be  seen  that  all  sensory  fibers 
are  centripetal  or  afferent,  but  not  all  centripetal  or  afferent 
fibers  are  sensory.  For  impressions  made  upon  the  terminations, 
or  upon  the  trunk,  of  a  centripetal  nerve  may  cause  (i)  pain,  or 
some  other  kind  of  sensation;  (2)  special  sensation,  (3)  reflex 
action  of  any  kind;  (4)  inhibition.  Similarly  impressions  made 
upon  a  centrifugal  nerve  may  (i)  cause  contraction  of  a  muscle 
(motor  nerve) ;  (2)  influence  nutrition  (trophic  nerve) ;  (3)  con- 
trol secretion  (secretory  nerve);  (^inhibit,  augment,  or  stop  any 
other  efferent  action  (Kirkes). 

To  these  two  classes,  efferent  and  afferent,  should  be  added  a 
third,  the  intercentral  fibers  which  connect  different  parts  of  the 
nervous  centers.  Most  of  these  even  can  be  called  either  afferent 
or  efferent. 

Characteristics  of  Efferent  Nerves. — In  case  of  these  nerves 
a  force  is  generated  in  the  centers  and  conveyed  by  the  nerves 
to  the  periphery,  where  it  manifests  itself  in  one  of  the  ways 
mentioned  above  as  characteristic  of  centrifugal  fibers. 
Division  of  these  fibers,  or  interference  with  their  conductivity 
by  disease  or  otherwise,  renders  impossible  the  manifestation  of 
nervous  force  generated  in  the  center,  for  the  simple  reason  that 
the  organ  to  which  the  fibers  are  distributed  cannot  receive  the 
message  intended  for  it.  For  instance,  a  muscle  cannot,  by  the 
most  persistent  effort  of  the  will  be  made  to  contract  if  the  motor 
fibers  running  to  that  muscle  are  divided.  In  case,  however,  of 
division  of  efferent  nerves,  if  the  peripheral  end  be  irritated,  thus 
roughly  counterfeiting  normal  stimulation,  the  ordinary  effects 
of  normal  stimulation  will  be  brought  about,  provided  (as  is 
usually  the  case)  that  particular  nerve  can  be  thus  directly  stimu- 
lated. Stimulation,  however,  of  the  central  end  of  such  a  cut 
nerve  produces  no  effect.  No  matter  whether  such  efferent 
nerves  receive  their  stimulus  directly  from  the  center  or  arti- 
fically,  as  by  mechanical  or  electrical  means,  the  effect  is  pro- 
duced in  the  end  organs,  whatever  they  may  be.  It  is  an  invari- 


232  THE   NERVOUS    SYSTEM 

able  law  to  which  reference  has  already  been  made,  that  a  nerve 
fiber  thus  conducting  a  message  in  either  direction  is  not  inter- 
fered with  by  the  proximity  of  other  fibers,  similar  or  dissimilar. 
Such  message  is  not  in  any  way  imparted  to  a  neighboring  fiber 
or  diffused  through  the  fasciculus,  but  is  conveyed  uninterrupt- 
edly to  its  destination.  It  is  possible  that  the  myeline  sheath 
has  an  insulating  effect  upon  the  contained  axis  cylinder,  just  as 
an  electric  wire  may  be  insulated  by  non-conducting  substances 
like  silk,  but  this  is  doubtful. 

Interesting  manifestations  of  motor  centrifugal  impulses  are 
seen  in  certain  movements  associated  with  corresponding  mus- 
cles on  different  sides  of  the  body  and  with  sets  of  muscles  on  the 
same  side.  It  is  almost  impossible  to  effect  certain  movements 
with  a  single  finger  or  toe  without  causing  similar  movements  in 
other  fingers  and  toes;  a  part  of  a  muscle  cannot  be  made  to 
contract  separately;  it  is  doubtful  if  it  be  possible  to  move  one 
eye-ball  without  the  other,  even  by  the  most  persistent  practice. 
Other  similar  examples  are  numerous.  It  is  quite  probable  that 
in  most  cases  these  associated  movements  are  solely  matters  of 
habit.  But  the  connection  by  commissural  fibers  of  the  cells  in 
the  centers  controlling  and  regulating  the  movement  of  these 
muscles  and  sets  of  muscles  would  offer  a  not  unreasonable  ex- 
planation of  the  phenomena  in  question,  since  such  an  arrange- 
ment might  render  impossible  separate  and  individual  action  by 
the  cells  thus  connected.  Excepting,  perhaps,  the  movements 
of  the  eye-balls,  these  associated  movements  can  be  greatly 
modified  by  education. 

Characteristics  of  Afferent  Nerves. — Impressions  received 
by  these  fibers,  although  they  are  conveyed  toward  the  center  and 
must  reach  a  center  before  there  is  any  nervous  manifestation,  are 
always  referred  to  the  periphery.  A  most  common  illustration  of 
this  fact  is  furnished  by  injury  to  the  ulnar  nerve  as  it  passes  the 
elbow — such  injury  being  manifested  not  usually  by  any  pain  at 
the  point  of  infliction,  but  on  the  ulnar  side  of  the  hand  where 


AFFERENT   NERVES  233 

the  nerve  is  distributed.  A  person  whose  limb  has  been  ampu- 
tated often  seems  to  feel  pain  in  the  extremity  although  it  has 
been  removed  from  the  body — such  pain  coming  from  compres- 
sion by  the  cicatrix  (or  otherwise)  of  the  nerves  which  before 
the  amputation  were  distributed  to  the  severed  limb.  Here,  as 
in  the  case  of  efferent  nerves,  division  of  the  fibers  between  the 
seat  of  impression  and  the  center  precludes  the  possibility  of 
any  nervous  manifestation.  That  is  to  say,  no  pain  will  be  felt, 
no  matter  how  great  the  injury  be,  if  the  sensory  fibers  running 
from  the  seat  of  injury  be  divided.  Stimulation  of  the  periph- 
eral end  of  a  divided  afferent  fiber  produces  no  effect;  but  stimu- 
lation of  the  central  end  is  followed  by  the  ordinary  manifestation 
— by  pain  if  the  nerve  stimulated  be  a  common  sensory  one. 
This  remark,  of  course,  applies  only  to  those  nerves  which  can 
be  thus  directly  stimulated — typically  to  true  sensory  fibers. 

Impressions  conveyed  by  nerves  of  special  sense  must  be  re- 
ceived through  the  intervention  of  certain  complex  organs,  con- 
sideration of  which  belongs  elsewhere. 

Although  a  division  has  been  made  of  nerve  fibers  into  afferent 
and  efferent,  each  with  definite,  proper  and  dissimilar  functions 
so  far  as  the  direction  of  conduction  is  concerned,  it  has  been 
impossible  to  discover  any  actual  difference  in  the  composition, 
appearance,  or  other  properties,  of  the  actual  fibers  themselves. 
In  fact,  it  may  be  even  considered  as  only  an  accident  that  one 
fiber  conveys  a  message  peripherally  and  another  centrally — an 
accident  dependent  upon  the  kind  of  center  which  with  the  fiber 
is  connected  and  the  kind  of  termination  it  has  in  the  periphery. 

Direction  of  the  Current  in  Nerve  Fibers.— It  has  long  been 
understood  that  in  no  case  will  a  fiber  in  situ  convey  a  message  at 
one  time  in  one  direction  and  at  another  in  an  opposite  one,  that 
no  individual  fiber  can  be  both  afferent  and  efferent;  and  so  far  as 
practical  action  is  concerned  this  is  true,  but  "experiment  has 
shown  that  if  a  nerve  trunk  be  stimulated  at  a  given  point,  then 
the  nerve  impulse  can  be  demonstrated  as  passing  away  from  the 


234  THE   NERVOUS    SYSTEM 

point  of  stimulation  in  both  directions"  (American  Text-book). 
However,  only  the  message  traveling  in  the  physiological  direc- 
tion is  manifest,  for  it  is  the  only  one  which  finds  a  suitable 
terminal. 

It  is  not  to  be  concluded,  however,  that  in  any  nerve  trunk,  as 
the  ulnar  nerve,  there  may  not  be  both  afferent  and  efferent 
fibers.  Such,  in  fact,  is  the  usual  arrangement.  Any  nerve 
trunk  may  contain  all  kinds  of  fibers — sensory,  special  sensory, 
vaso-motor,  motor,  trophic,  secretory — but  the  presence  of  all 
these  does  not  interfere  with  the  individuality  and  the  individual 
action  of  each  fiber.  A  nerve  trunk  containing  more  than  one 
kind  of  fibers  is  called  a  mixed  nerve. 

Speed  of  Nervous  Conduction. — It  is  stated  that  afferent  im- 
pressions are  conveyed  by  nerves  at  the  rate  of  about  120  feet 
per  second;  the  rate  for  efferent  impulses  is  somewhat  less  rapid, 
probably  no  feet.  In  the  spinal  cord  tactile  impressions  are 
conveyed  a  little  faster  than  in  the  nerves  proper,  and  painful 
impressions  somewhat  less  than  one-half  as  fast.  The  rate  of 
motor  conduction  in  the  cord  is  said  to  be  one-third  the  rate  in 
the  nerves.  It  has  also  been  demonstrated  that  an  act  of  volition 
requires  a  definite  time  for  the  inception  of  its  performance;  this 
is  stated  to  be  about  -fa  of  a  second.  The  recognition  of  a  simple 
impression  (conveyed  in  the  opposite  direction,  of  course)  re- 
quires about  2^  of  a  second.  Furthermore,  the  part  played  by 
the  spinal  cord  in  reflex  action  (to  be  considered  later)  also  con- 
sumes an  appreciable  period;  this  is  found  to  be  more  than 
twelve  times  the  period  occupied  in  the  transmission  of  the 
impression  to  the  cord  or  the  impulse  back  to  the  muscles. 

Action  of  Electricity  Upon  Nerves. — A  nerve  may  be  irri- 
tated in  any  one  of  several  ways;  but  mechanical,  thermal  and 
chemical  irritants,  besides  working  injury  to  the  tissues,  are  much 
less  easily  managed  and  regulated  than  is  electricity.  This 
agent  may  be  applied  time  after  time  to  a  nerve  trunk  without 
causing  any  permanent  change  in  its  conductivity,  and  the 


THE    CEREBRO-SPINAL  AXIS  235 

strength,  time  and  duration  of  application,  etc.,  can  be  accurately 
governed. 

It  has  been  noticed  that  the  uninterrupted  flow  of  an  electric 
current  through  a  nerve  is  unattended  by  muscular  contraction ; 
it  has  likewise  been  seen  that  very  slow  changes  in  the  strength 
of  the  current  are  similarly  unaccompanied  by  the  manifestations 
of  ordinary  stimulation;  but  sudden  changes  in  the  strength, 
whether  in  the  direction  of  increase  or  decrease,  act  as  stimuli. 
However,  while  the  passage  of  a  constant  current  through  a  nerve 
does  not  manifest  itself  by  contractions  except  at  making  and 
breaking,  such  a  passage  brings  about  a  change  in  the  tissue  of  the 
nerve  known  as  electrotonus.  It  may  be  considered  a  state  of 
electric  tension.  In  the  anodic  area  the  excitability  is  diminished 
(anelectrotonus) ;  in  the  kathodic  area  it  is  increased  (katelectro- 
tonus).  Nor  is  the  electrotonic  condition  restricted  to  that  por- 
tion of  the  nerve  between  the  poles.  Between  the  poles  there  is  a 
point  where  the  two  influences — anelectrotonus  and  katelectro- 
tonus — meet  and  there  is  neither  increased  nor  decreased  excit- 
ability. With  weak  currents  this  point  is  nearer  the  anode; 
with  strong  ones  nearer  the  kathode.  A  descending  current 
diminishes  the  excitability  of  a  nerve;  an  ascending  increases  it. 
Prolonged  application  of  electric  stimuli  will  exhaust  nervous 
excitability,  but  it  may  be  restored  by  rest,  or  more  quickly  by  an 
opposite  current. 

THE  CEREBRO-SPINAL  AXIS. 

The  cerebro-spinal  axis  embraces  the  nervous  matter  in  the 
cranial  cavity  and  in  the  spinal  canal,  excepting  the  roots  of  the 
cranial  and  spinal  nerves.  This  axis  consists  of  both  white  and 
gray  matter.  The  white  matter  is  made  up  of  conducting  ele- 
ments; the  gray  matter  consists  of  a  number  of  connected  ganglia. 
In  the  cord  the  white  matter  is  situated  externally;  in  the  brain 
the  gray.  The  encephalon  is  situated  in  the  cranial  cavity 
and  consists  of  the  cerebrum,  the  cerebellum,  the  pons  Varolii, 


236  THE    NERVOUS    SYSTEM 

and  the  medulla  oblongata.  These  different  parts  are  con- 
nected with  each  other  and  with  the  cord  by  nerve  fibers,  and  all 
the  cranial  and  spinal  nerves  are  connected  with  gray  matter 
either  in  the  brain  or  in  the  cord,  or  in  both.  This  gray  matter 
exists  for  the  purpose  of  receiving  impressions  and  generating 
nerve  force. 

Membranes. — The  encephalon  and  cord  are  covered  by  mem- 
branes for  protection  and  for  the  support  of  vessels  belonging 
thereto.  These  are  (i)  the  dura  matter,  (2)  the  arachnoid  and 
(3)  the  pia  mater. 

The  dura  mater  is  a  dense  fibrous  structure  surrounding  the 
encephalon  and  adherent  to  the  inner  surfaces  of  the  cranial 
bones.  At  certain  points  the  two  layers  of  which  it  is  composed 
separate  to  form  the  venous  sinuses.  Processes  of  the  internal 
layers  also  are  sent  inward  between  the  two  lobes  of  the  cere- 
brum (falx  cerebri),  between  the  cerebrum  and  cerebellum  (ten- 
torium  cerebelli  and  between  the  lateral  halves  of  the  cerebellum 
(falx  cerebelli).  This  membrane  passes  through  the  foramen 
magnum  to  cover  also  the  spinal  cord,  and  to  follow  as  a  sheath 
the  spinal  nerves  at  their  foramina  of  exit. 

The  arachnoid  resembles  the  serous  membranes.  It  covers 
the  brain  and  cord  underneath  the  dura  mater  without  dipping 
into  the  sulci  of  the  brain.  Between  it  and  the  pia  mater  is 
what  is  known  as  the  subarachnoid  space  containing  the  sub- 
arachnoid  fluid.  This  fluid  serves  a  mechanical  purpose,  equal- 
izing pressure  in  different  parts  of  the  cerebro-spinal  axis  and 
protecting  the  nervous  substance  from  injury  by  concussion,  etc. 
Besides  being  found  in  the  subarachnoid  space,  it  occupies  the 
ventricles  of  the  brain  and  the  central  canal  of  the  cord,  com- 
munication between  these  being  furnished  by  a  small  opening  at 
the  inferior  angle  of  the  floor  of  the  fourth  ventricle. 

The  pia  mater  is  a  very  delicate  structure  dipping  between  the 
convolutions  of  nervous  matter  and  lying  in  close  contact  with 
the  external  surface  of  the  encephalon  and  cord.  It  is  exceed- 


THE    SPINAL    CORD 


237 


ingly  vascular;  indeed  its  main  function  is  to  support  vessels  be- 
longing to  the  nervous  substance  underneath.  Both  the  arach- 
noid and  the  pia  mater  pass  out  at  the  foramen  magnum  with 
the  dura  to  cover  the  cord. 

The  Spinal  Cord. 

The  spinal  cord  occupies  the  spinal  canal  and  is  about  eight- 
een inches  long,  extending  from  the  foramen  magnum  to  the 
lower  border  of  the  first  lumbar  vertebra.  Its  distal  extremity  is 


FIG.  73. —Different  views  of  a  portion  of  the  spinal  cord  from  the  cervical 
region,  with  the  roots  of  the  nerves.     (Slightly  enlarged.) 

In  A,  the  anterior  surface  of  the  specimen  is  shown;  the  anterior  nerve  -root  of  its 
right  side  is  divided;  in  B,  a  view  of  the  right  side  is  given;  in  C,  the  upper  surface  is 
shown;  in  D,  the  nerve-roots  and  ganglion  are  shown  from  below,  i,  the  anterior 
median  fissure;  2,  posterior  median  fissure;  3,  anterior  lateral  depression,  over  which 
the  anterior  nerve-roots  are  seen  to  spread;  4,  posterior  lateral  groove,  into  which 
the  posterior  roots  are  seen  to  sink;  5,  anterior  roots  passing  the  ganglion;  5',  in  A, 
the  anterior  root  divided;  6,  the  posterior  roots,  the  fibers  of  which  pass  into  the 
ganglion  6';  7,  the  united  or  compound  nerve;  7',  the  posterior  primary  branch,  seen 
in  A  and  D  to  be  derived  in  part  from  the  anterior  and  in  part  from  the  posterior 
root.  (Kirkes  after  Allen  Thomson.) 


238  THE   NERVOUS    SYSTEM 

in  the  shape  of  a  slender  filament  known  as  filum  lerminale, 
which  is  gray  in  color.  The  sacral  and  coccygeal  nerves,  having 
taken  origin  from  the  cord  in  the  dorsal  region,  pass  downward 
in  the  canal  to  find  exit  through  the  sacral  and  coccygeal  fora- 
mina. This  collection  of  nerves  thus  passing  down  is  known  as 
the  cauda  equina. 

Gross  Divisions  of  the  Spinal  Cord  in  Section.— Cross  sec- 
tion of  the  cord  reveals  the  division  of  its  substance  into  two 
lateral  halves  connected  by  the  anterior  and  posterior  commissures. 
In  the  center  of  the  cord,  and  between  these  commissures,  is  a 
small  opening,  the  central  canal  of  the  cord,  communicating  with 
the  fourth  ventricle  above.  This  division  of  the  substance  of  the 
cord  into  lateral  halves  is  effected  by  the  two  median  fissures, 
anterior  and  posterior.  The  former  is  the  more  clearly  marked, 
and  is  lined  thoughout  with  pia  mater.  It  is  bounded  pos- 
teriorly by  the  anterior  white  commissure.  The  posterior  median 
fissure  is  not  lined  with  pia  mater  and  extends  anteriorly  as  far 
as  the  posterior  gray  commissure.  It  is  to  be  noted  that  there 
are  both  anterior  and  posterior  gray  commissures,  but  only  one 
white  commissure  (anterior),  which  is  bounded  posteriorly  by 
the  anterior  gray  commissure. 

Besides  the  anterior  and  posterior  median  fissures  there  are 
also  on  each  side  antero-lateral  and  postero-lateral  fissures,  mark- 
ing the  lines  of  exit  of  the  anterior  and  posterior  roots  of  the 
spinal  nerves.  These  are  not  well  defined.  They  divide  the 
cord  into  anterior,  posterior  and  two  lateral  columns. 

Arrangement  of  Gray  Substance. — The' disposition  of  the 
gray  substance  in  the  cord  (in  transverse  section)  is  somewhat 
after  the  manner  of  the  letter  H,  each  lateral  portion  represent- 
ing the  anterior  and  posterior  cornua  of  gray  matter  for  that  side, 
and  being  connected  to  the  corresponding  portion  of  the  other 
side  by  the  commissures  embracing  the  central  canal.  The 
anterior  cornua  are  shorter  and  thicker  than  the  posterior. 
From  these  issue  the  anterior  and  posterior  roots  respectively  of 


THE    SPINAL   CORD  239 

the  spinal  nerves.  The  cells  are:  (i)  Those  in  the  anterior  cornu; 
(2)  those  in  the  posterior  cornu;  (3)  those  in  the  lateral  aspect 
of  the  gray  matter;  (4)  those  at  the  inner  base  of  the  posterior 
cornu  (Clarke's  vesicular  column). 

The  gray  substance  is  made  up  of  cells  with,  of  course,  the 
usual  neuroglia  and  blood-vessels.  The  cells  in  the  anterior 
cornua  are  large  in  size  and  possess  a  greater  number  of  poles 
than  those  in  the  posterior  cornua;  from  their  connection  with  the 
anterior  (motor)  spinal  nerve  roots  they  are  called  motor  cells 
in  contradistinction  to  the  sensory  cells  in  the  posterior  cornua 
which  are  connected  indirectly  with  the  posterior  (sensory)  nerve 
roots. 

Degeneration. — Nerve  fibers  when  separated  from  the  cells 
of  which  they  are  outgrowths  degenerate.  Fibers  have  been 
said  to  degenerate  in  the  direction  in  which  they  carry  messages, 
but  this  is  by  no  means  always  so.  For  instance,  the  parent  cells 
for  the  fibers  of  the  posterior  spinal  roots  are  in  the  ganglia  on 
those  roots  near  the  cord,  and  section  of  the  root  beyond  the 
ganglion  causes  degeneration  of  its  fibers  peripherally — which 
is  in  the  opposite  direction  to  the  passage  of  impressions  in  them. 
Section  of  the  posterior  root  between  the  ganglion  and  cord  is 
followed  by  centripetal  degeneration,  and  there  is  no  centrifugal 
degeneration.  The  anterior  spinal  root  fibers  are  outgrowths  of 
cells  in  the  anterior  cornua  of  gray  matter.  Section  of  this  root 
anywhere  occasions  centrifugal  degeneration  (Fig  74) . 

Arrangement  of  the  White  Substance.— It  is  scarcely 
necessary  to  state  that  the  white  substance  of  the  cord  consists  of 
nerve  fibers  with  their  usual  accompaniments.  It  is  external  to 
the  gray.  The  fibers  are  medullated,  but  have  no  sheath  of 
Schwann. 

The  divisions  of  the  cord  already  referred  to  are  purely  ana- 
tomical. Physiological  and  pathological  researches  warrant  the 
further  division  of  the  white  substance  of  the  cord  into  eight 
columns  for  each  side.  The  course  of  all  the  fibers  in  the  white 


240 


THE   NERVOUS    SYSTEM 


matter  of  the  cord  is  by  no  means  certain.  The  division  here 
given  may  not  be  strictly  correct,  but  it  probably  receives  as 
little  adverse  criticism  as  any  of  the  others.  Classified  accord- 
ing to  the  direction  in  which  their  fibers  degenerate  after  section 
the  paths  are:  (i)  Degenerating  downward,  (a)  the  column  of 
Turck  and  (b)  the  crossed  pyramidal  tract;  (II)  degenerating 
upward,  (a)  the  column  of  Goll  and  (b)  the  direct  cerebellar 


FIG.    74. — Diagram   to   illustrate   wallerian   degeneration   of   nerve-roots. 

(Kirkes.) 

tract;  (III)  degenerating  in  neither  direction,  (a)  the  anterior 
fundamental  fasciculus,  (b)  the  anterior  radicular  zone,  (c)  the 
mixed  lateral  column  and  (d)  the  column  of  Burdach. 

I.  (a)  The  column  of  Turck  occupies  a  position  just  lateral  to 
the  anterior  median  fissure  and  extends  downward  to  the  lower 
dorsal  region.  Its  fibers  decussate  high  up  in  the  cord.  This 
column  is  sometimes  called  the  direct,  or  uncrossed,  pyramidal 
tract,  as  distinguishing  it  from  the  other  descending  column. 
(b)  The  crossed  pyramidal  tract  is  external  to  the  posterior  cornu 
of  gray  matter  and  internal  to  the  direct  cerebellar  tract.  Its 


THE    SPINAL   CORD  241 

fibers    decussate    in    the    anterior    pyramids    of    the    medulla 
oblongata. 

II.  (a)  The  direct  cerebellar  tract  occupies  the  outer  posterior 
part  of  the  lateral  column.  Its  fibers  reach  the  cerebellum 
through  the  inferior  peduncles,  after  having  traversed  the  poste- 
rior pyramids  of  the  medulla.  This  tract  exists  throughout  the 
length  of  the  coid.  (V)  The  column  of  Goll  (postero-internal 
column)  is  situated  posteriorly  in  a  position  corresponding  to 


a 


FIG.  75. — Scheme  of  the  conducting  paths  in  the  spinal  cord  at  the  3d  dorsal 

nerve. 

The  black  part  is  the  gray  matter.  V,  anterior,  hw,  posterior  root;  a,  direct,  and  g, 
crossed,  pyramidal  tracts;  b,  anterior  fundamental  fasciculus;  c,  Goll's  column;  d, 
column  of  Burdach;  e,  anterior  radicular  zone;  f,  mixed  lateral  tract;  h,  direct  cere- 
bellar tracts.  (Landois,  modified.) 

the  column  of  Turck  anteriorly — just  lateral  to  the  posterior 
median  fissure.  Fibers  in  this  column  extend  from  the  upper 
lumbar  region  to  the  funiculi  graciles  of  the  medulla. 

III.  (a)  The  anterior  fundamental  fasciculus  lies  between  the 
column  of  Turck  internally  and  the  anterior  cornu  and  anterior 
roots  of  the  spinal  nerves  externally.  Its  fibers  are  lost  in  the 
medulla  above,  (b)  The  anterior  radicular  zone  is  external  to 
the  anterior  roots  of  the  spinal  nerves  and  anterior  to  the  crossed 
pyramidal  tract  and  the  direct  cerebellar  fasciculus.  Its  fibers 
are  lost  in  the  medulla  above,  (c)  The  mixed  lateral  column  is 
16 


242  THE   NERVOUS    SYSTEM 

just  external  to  the  main  body  of  gray  matter  and  does  not  reach 
the  surface  of  the  cord.     Its  fibers  are  likewise  lost  in  the  medulla 


a/I* 

FIG.  76. — Course  of  the  fibers  for  voluntary  movement. 

ab,  path  for  the  motor  nerves  of  the  trunk;  c,  fibers  of  the  facial  nerve;  B,  corpus 
callosum;  Nc,  nucleus  caudatus;  G.  i,  internal  capsule;  N.  I,  lenticular  nucleus;  P, 
pons;  N.f,  origin  of  the  facial;  Py,  pyramids  and  their  decussation;  Ol,  olive.  Gr, 
restiform  body;  PR,  posterior  root;  AR,  anterior  root;  x,  crossed,  and  z,  direct 
pyramidal  tracts.  (Landois.) 

oblongata.  (d)  The  column  of  Burdach  (postero-external  col- 
umn) is  situated  posteriorly  in  a  location  corresponding  to  the 


MOTOR   PATHS   IN   THE   CORD  243 

anterior  fundamental  fasciculus  anteriorly — external  to  the 
column  of  Goll  and  internal  to  the  posterior  cornu.  Its  fibers 
reach  the  cerebellum  through  the  inferior  peduncles,  having 
passed  through  the  restiform  bodies. 

Functions  of  the  Columns. — Remarks  already  made  touch- 
ing the  direction  of  degeneration  in  the  separate  columns  throw 
some  light  upon  the  physiological  function  of  the  fibers  in  each. 

Motor  impulses  pass  downward  from  the  brain  through  cer- 
tain fibers  to  the  cells  of  the  anterior  cornua  of  gray  matter  in 
the  cord,  and  are  sent  thence  through  the  spinal  nerves  to  the 
muscles.  The  paths  in  the  cord  conveying  these  impulses  are 
found  to  be  the  columns  of  Turck  and  the  crossed  pyramidal 
tracts,  and  these  are  the  only  parts  of  the  cord  known  so  to  act. 
Impulses  to  the  upper  segment  of  the  cord  may  be  conveyed  by 
either  of  these  columns,  but  impulses  to  the  lower  segment  must 
follow  the  crossed  pyramidal  tract,  since  the  column  of  Turck 
ceases  to  exist  in  the  dorsal  region.  Only  some  3-7  per  cent, 
of  motor  fibers  from  the  cortex  are  thought  to  enter  the  columns 
of  Turck.  The  others  decussate  in  the  medulla  and  enter  the 
crossed  pyramidal  tracts.  In  any  case  motor  impulses  originat- 
ing in  the  brain  and  so  conveyed  are  manifested  on  the  side 
opposite  their  cerebral  origin,  since  the  fibers  in  both  these 
tracts  decussate  in  passing  downward.  It  is  a  well  known  patho- 
logical fact  that  lesions  of  motor  areas  in  the  brain,  or  section  of 
one  lateral  half  of  the  cord,  are  followed  by  paralysis  on  the  side 
opposite  the  lesion. 

Following  a  motor  fiber  (A,  Fig.  77)  through  the  anterior  root 
of  a  spinal  nerve,  it  is  found  to  originate  from  one  of  the  large 
multipolar  cells  (3)  in  the  anterior  cornu  of  gray  matter.  Around 
these  anterior  horn  cells  (i,  2,  3,  4)  arborize  the  end  filaments 
of  fibers  which  have  come  down  through  the  cord  from  the 
brain.  Some  fibers  have  come  down  in  the  uncrossed  pyramidal 
tract  (column  of  Turck)  on  the  side  opposite  the  cells,  i,  2,  3,  4, 
and  crossed  over  to  the  same  side  through  the  anterior  white 


244 


THE   NERVOUS    SYSTEM 


commissure  approximately  on  a  level  with  the  cells;  others  have 
decussated  in  the  medulla,  and  come  down  in  the  crossed  pyram- 
idal tract  on  the  same  side  as  the  cells.  In  both  cases  the  fibers 
originated  in  the  brain  on  the  side  opposite  the  cells  around  which 


FIG.  77.— Course  of  nerve  fibers  in  spinal  cord.     (Kirkes  after  Schafer.} 


they  arborize  in  the  cord.     This  is  the  connection  which  exists 
between  the  brain  and  the  anterior  root  fibers. 

Not  all  fibers  in  the  anterior  nerve  roots  are  thus  prolonged 
upward  in  the  pyramidal  tracts.  The  number  of  fibers  in  these 
roots  is  much  larger  than  in  the  pyramidal  tracts,  and  conse- 
quently some  of  them  must  end  (originate)  directly  in  the  cells 
of  the  anterior  cornua.  Furthermore,  it  seems  that  some  fibers 


SENSORY  PATHS  IN  THE  CORD  245 

pass  from  the  anterior  nerve  roots  directly  into  the  pyramidal 
tracts  without  being  interrupted  by  motor  cells. 

The  column  of  Turck  and  the  crossed  pyramidal  tract  are, 
therefore,  the  motor  paths  in  the  cord. 

Fibers  entering  the  cord  by  the  posterior  roots  send  prolonga- 
tions both  upward  and  downward  in  the  gray  matter  of  the  cord, 
and  communicate  by  end  arborizations  with  the  small  sensory 
cells  in  the  posterior  cornua  and  with  cells  in  several  other 
localities.  (See  Figs.  77,  84.)  Reference  to  Fig.  77  will  show 
that  the  connection  of  the  anterior  nerve  fibers  with  the  gray 
matter  of  the  cord  is  simple,  while  that  of  the  posterior  is  com- 
paratively complex,  i,  2,  3,  4  are  anterior  horn  cells.  Each  of 
these  gives  rise  to  an  efferent  fiber,  one  of  which  (A )  is  shown 
distributed  to  a  muscle  (M).  Each  of  these  cells  also  is  sur- 
rounded by  the  end  arborization  of  a  fiber  (P)  from  the  cortex. 

A  fiber  from  the  posterior  root  is  also  shown.  It  originates 
in  a  cell  of  the  sensory  ganglion  (G).  It  bifurcates,  one  branch 
going  to  the  surface  (S),  the  other  enters  the  cord  and  itself  bi- 
furcates. The  branch  (E)  is  short  and  arborizes  around  a  small 
cell  (Pj)  in  the  posterior  cornu,  from  which  a  new  axis  cylinder 
arises  to  arborize  around  the  anterior  horn  cell  (4) .  The  other 
branch  (D)  travels  upward  in  the  posterior  column  of  the  cord. 
A  collateral  (5)  is  seen  going  to  the  anterior  horn  cell  (2),  one  to 
the  posterior  horn  cell  (P2)  and  another  to  a  cell  (C)  in  the 
inner  base  of  the  posterior  cornu  (in  Clarke's  column);  from 
C  an  axis  cylinder  enters  the  direct  cerebellar  tract.  The 
main  fiber  (8)  may  terminate  in  the  gray  matter  of  the  cord 
above,  or  in  the  medulla.  Impressions  brought  thus  to  the 
cord  are  carried  to  the  opposite  side  and  pass  up  through  the 
gray  matter  in  most  part.  The  fibers  decussate  at  no  particular 
point,  but  throughout  the  length  of  the  cord.  However,  some 
fibers  bearing  sensory  impressions  pass  to  the  column  of  Goll 
and  thus  upward,  while  some  also  go  to  the  encephalon  by  the 
direct  cerebellar  fasciculi  and  the  columns  of  Burdach.  Ex- 


246 


THE   NERVOUS    SYSTEM 


perimentally,  decussation  of  sensory  fibers  is  demonstrated  (i)  by 
longitudinal  section  of  the  spinal  cord  in  the  median  line,  which 
is  followed  by  anesthesia  on  both  sides  below  the  section;  and  (2) 
by  horizontal  section  of  one-half  of  the  cord,  which  is  followed 


FIG.  78. — Transverse  section  through  half  the  spinal  cord,  showing    the 

ganglia. 

A,  anterior  cornual  cells;  B,  axis-cylinder  process  of  one  of  these  going  to  posterior 
root;  C,  anterior  (motor)  root;  D,  posterior  (sensory)  root;  E,  spinal  ganglion  on 
posterior  root;  F,  sympathetic  ganglion;  G,  ramus  communicans;  H,  posterior  branch 
of  spinal  nerve;  I,  anterior  branch  of  spinal  nerve;  a,  long  collaterals  from  posterior 
root  fibers  reaching  to  anterior  horn;  b,  short  collaterals  passing  to  Clarke's  column; 
c,  cell  in  Clarke's  column  sending  an  axis-cylinder  process  (d)  to  the  direct  cerebellar 
tract;  e,  fiber  of  the  anterior  root;/,  axis  cylinder  from  sympathetic  ganglion  cell, 
dividing  into  two  branches,  one  to  the  periphery,  the  other  toward  the  cord;  g,  fiber 
of  the  anterior  root  terminating  by  an  arborization  in  the  sympathetic  ganglion;  h, 
sympathetic  fiber  passing  to  periphery.  (Kirkes  after  Ramony  Cajal.) 


by  anesthesia  on  the  opposite  side  below  the  section.  It  is 
claimed  that  pain  and  temperature  sensations  decussate  at  once 
on  reaching  the  gray  matter,  while  sensations  of  touch,  pressure 
and  equilibration  pass  up  on  the  same  side  until  the  medulla  is 


FUNCTIONS    OF   THE    SPINAL   CORD  247 

reached.  Some  afferent  fibers  are  probably  not  continued 
upward  to  the  brain  either  directly  or  indirectly. 

It  thus  appears  that  we  have  no  very  accurate  knowledge  of 
the  sensory  paths  in  the  cord.  The  gray  matter  seems  princi- 
pally concerned;  but  the  columns  of  Goll  and  Burdach  and 
the  direct  cerebellar  fasciculi  also  convey  afferent  impressions. 

The  columns  of  Burdach  have  been  said  to  present  no  degen- 
eration secondary  to  section.  Trophic  centers  for  their  fibers 
must,  therefore,  exist  above  and  below  any  given  point  of  section. 
It  is  found  that  the  fibers  constituting  these  columns  pass  in  and 
out  along  the  cord  between  cells  in  different  planes  and  acting 
as  longitudinal  commissural  fibers.  In  locomotor  ataxia  the 
characteristic  symptom  is  inability  to  coordinate  the  muscular 
movements — especially  of  the  lower  extremities;  the  characteristic 
lesion  has  been  found  to  be  in  the  columns  of  Burdach. 
This  is  of  importance  in  determining  the  function  of  these  col- 
umns, and,  in  fact,  leads  to  the  conclusion  that  their  fibers  assist 
in  regulating  and  coordinating  the  voluntary  movements.  This 
opinion  is  further  supported  by  the  connection  of  these  fibers  with 
the  cerebellum,  which  contains  the  center  for  muscular  coordina- 
tion— if  such  a  center  exist.  The  sense  of  pressure  and  the  so- 
called  muscular  sense  are  probably  connected  with  the  fibers  of 
this  column,  and  these  may  be  the  only  sensory  impressions 
conveyed  through  the  columns  of  Burdach. 

The  anterior  fundamental  fasciculi,  the  anterior  radicular 
zones,  and  the  mixed  lateral  paths  degenerate  in  neither 
direction  after  section,  their  trophic  cells  existing  at  both  extremi- 
ties. They  connect  cells  in  the  gray  matter  of  the  cord. 

Functions  of  the  Spinal  Cord. — These  are  (i)  conductions, 
(2)  transference,  (3)  reflex  action,  (4)  augmentation,  (5)  co- 
ordination, (6)  inhibition  of  reflex  acts,  (7)  special  centers  (Collin 
and  Rockwell,  modified). 

i.  Conduction. — This  has  been  referred  to  in  discussing   the 


248  THE   NERVOUS    SYSTEM 

white  columns  of  the  cord.  This  function  makes  it  possible  for 
the  brain  to  receive  impressions  from  and  send  impulses  to  the 
periphery.  It  is  to  be  remembered  that  most  of  these  impres- 
sions and  impulses  are  interrupted  by  spinal  nerve  cells  in  their 
passage  between  brain  and  periphery. 

2.  Transference. — An  impression  reaching  the  gray  matter  of 
the  cord  may  be  transferred  (not  as  in  typical  reflex  action)  so 
as  to  be  felt  in  an  entirely  different  region  from  that  in  which 
the  irritation  takes  place.     Hip  joint  disease  often  gives  pain 
in  the  knee  alone. 

3.  Reflex  Action. — The  cord  may  act  as  a  center  without  the 
cooperation  of  the  brain.     Indeed,  by  no  means  do  muscular 
movements  cease  immediately  on  removal  of  the  encephalon  if 
the  cord  and  its  nerves  be  left  intact.     An  animal  so  mutilated 
possesses  no  sensation  or  volition,  but  for  a  time  the  sensory 
nerves  will  continue  to  convey  impressions  and  the  motor  nerves 
impulses.     Under  these  conditions  impressions  (as  of  heat)  are 
conveyed  to  the  cord  by  the  afferent  nerves;  the  gray  matter  of 
the  cord  receives  the  impressions  and   generates  motor   force 
which  is  sent  out  through  the  corresponding  efferent  nerves,  and 
movements  result.     This  is  reflex  action.     The  impression  is 
reflected  through  the  cord  and  manifested  in  motion  without  the 
intervention  of  sensation  or  volition.     Reference  to  Figs.  77  and 
80  shows  how  reflex  action  is  anatomically  possible  through  the 
cord  connections.     Typical  reflex  action  requires  anatomically 
(i)  something  to  produce  an  impression,  (2)  a  nerve  terminal  to 
receive  it,  (3)  a  centripetal  fiber  to  convey  it,  (4)  a  center  to  re- 
ceive and  transform  it,  (5)  a  centrifugal  fiber  to  convey  it  to  the 
periphery  and  (6)  a  muscle  to  contract.     This  remark  applies  to 
reflex  action  connected  with  the  cord,  but  by  common  consent 
reflex  action  is  not  limited  to  the  cord  and  its  connections. 

If  reflex  action  be  defined  as  any  involuntary  manifestation  of 
nerve  force  consequent  upon  the  reception  of  an  impression  (gen- 
eral or  special)  by  a  nerve  center,  the  term  must  be  made  to 


REFLEX  ACTION  249 

include  such  phenomena  as  intestinal  peristalsis,  contraction  and 
dilatation  of  the  pupil,  certain  mental  operations,  etc.  In  reality 
most  reflex  acts  are  of  a  complex  nature,  involving  associated 
action  on  the  part  of  several  neurons  and  being  manifested  fre- 
quently at  several  points.  For  example,  a  foreign  body  in  the 
larynx  causes  reflexly  not  only  closure  of  the  glottis,  but  also  the 
convulsive  muscular  contractions  incident  to  coughing.  The 
realm  of  reflex  action  is  obviously  a  wide  one. 

It  may  be  said  that  ordinary  reflexes  are  usually  under  the 
direction  of  the  cord,  but  this  does  not  imply  that  the  brain  may 
not  be  concerned.  Pricking  the  sole  of  the  foot  of  a  sleeping 
person  will  cause  him  to  draw  up  his  leg  without  the  interven- 
tion of  consciousness.  Probably  were  he  awake  the  withdrawal 
would  still  be  a  reflex  but  he  would  certainly  be  conscious  of 
the  pain,  though  after  the  act  of  withdrawal  was  accomplished. 
Nor  is  reflex  action  by  any  means  limited  to  the  cerebro- 
spinal  system.  Either  of  the  two  systems,  or  both,  may  be 
concerned. 

Now  in  order  for  reflex  movements  to  occur,  there  must  be  a 
transference  of  impressions  received  by  sensory  cells  to  cells  cap- 
able of  giving  origin  to  motor  impulses.  The  cells  communi- 
cate by  their  collaterals,  which  may  be  short  or  long,  depending 
on  the  distance  between  the  cells  concerned.  Cells  in  the  gray 
matter  of  the  cord  are  "connected"  by  such  fibers,  and  they  run 
largely  in  the  white  matter  of  the  cord  joining  cells  on  different 
planes.  They  constitute  the  larger  part  of  the  anterior  funda- 
mental fasciculi,  the  anterior  radicular  zones,  and  the  mixed 
lateral  tracts,  and  it  is  these  paths  which  are  mainly  concerned  in 
reflex  action  of  the  cord. 

4.  Augmentation. — Sensory  fibers,  on  reaching  the  cord,  send 
prolongations  both  upward  and  downward  in  the  gray  matter. 
These  prolongations,  by  their  end  arborizations,  seem  to  com- 
municate indirectly  with  several  motor  cells.  In  the  simplest 
reflex  movements  connected  wi}h  the  spinal  cord  the  muscular 


250  THE   NERVOUS    SYSTEM 

activity  is  limited  to  the  area  corresponding  to  the  distribution 
of  the  afferent  nerve  which  has  been  irritated;  but  if  the  irrita- 
tion be  sufficiently  increased  other  muscles  in  the  same  locality, 
or  the  corresponding  muscles  on  the  opposite  side  of  the  body,  or 
even  the  whole  musculature,  may  be  thrown  into  action.  This 
is  explained  on  the  ground  that  under  favorable  conditions  of 
central  excitability,  strength  of  peripheral  irritation,  etc.,  the 
afferent  impression  is  disseminated  by  collaterals  throughout  a 
large  area  of  the  cord  (for  example),  and  a  large  number  of  effer- 
ent cells  are  made  to  discharge.  The  reflex  excitability  of  the 
cord  is  markedly  increased  by  the  administration  of  such  drugs 
as  strychnin.  An  animal  so  poisoned  will  be  thrown  into  the 
most  violent  convulsions  by  so  slight  a  sensory  impression  as 
a  simple  breath  of  air.  Removal  of  the  encephalon  in  inferior 
animals  also  exaggerates  reflex  excitability. 

5.  Coordination. — This  has  been  referred  to  under  the  columns 
of  Burdach.     Coordination  is  "a  repetition  of  ordinary  reflex 
acts  for  our  daily  lives."     No  effort  is  necessary  to  coordinate 
the  muscular  movements  of  deglutition,  respiration,  walking,  etc. 
These  movements  may  be  performed  when  the  cerebrum  is 
removed. 

6.  Inhibition  of  Reflex  Acts. — This  is  not  a  function  of  the 
cord  proper,  but  is  directed  by  the  cerebrum.     A  great  many 
reflex  movements  may  be  inhibited  by  an  act  of  the  will,  provid- 
ing always  they  are  due  to  contraction  of  striped  muscle.     The 
reflex  acts  of  coughing  or  sneezing,  or  those  resulting  from 
tickling,  for  example,  can  be  largely  controlled.     These  are 
usually  performed  as  reflex  cord  acts,  but  the  brain  may  evidently 
assert  its  superiority  over  the  cord  and  inhibit  them. 

7.  Special  Centers. — In  the  gray  matter  of  the  cord  are  found 
various  centers  for  distinct  acts  such  as  defecation,  parturition, 
micturition,  etc.     These  are  all  connected  with  each  other  and 
with  the  encephalon  and  obey  the  usual  laws  of  reflex  action. 


THE    ENCEPHALON  251 

THE  ENCEPHALON. 

The  encephalon  is  situated  within  the  cranial  cavity  and  is 
commonly  called  the  brain.  Its  gross  divisions  are  the  medulla 
oblongata,  the  pom  Varolii,  the  cerebellum,  and  the  cerebrum. 
All  the  other  divisions  are  in  a  measure  subordinate  to  the  cere- 
brum, though  each  division  has  individual  functions.  The 
human  brain  weighs  about  49^  ounces  in  the  male  and  about 
44  in  the  female. 

The  Medulla  Oblongata. 

Anatomy. — The  medulla  oblongata,  or  bulb,  joins  the  upper 
extremity  of  the  spinal  cord  and  extends  to  the  pons  above.  It 
has  a  pyramidal  shape,  lies  in  the  basilar  groove  of  the  occipital 
bone,  and  is  slightly  flattened  antero-posteriorly.  It  is  about  an 
inch  and  a  quarter  in  length,  half  an  inch  thick,  and  three-quar- 
ters of  an  inch  broad  above.  The  anterior  and  posterior  median 
fissures  of  the  cord  are  continued  upward  in  the  medulla;  the 
central  canal  terminates  in  the  inferior  angle  of  the  fourth  ven- 
tricle. The  anterior  columns  appear  to  be  continuous  with 
the  anterior  pyramids  of  the  medulla.  These  pyramids  are  situ- 
ated just  lateral  to  the  anterior  median  fissure.  The  innermost 
fibers  of  the  pyramids  are  the  continuations  upward  of  the  crossed 
pyramidal  tracts,  and  are  seen  to  decussate  in  the  median  line; 
the  outermost  fibers  are  the  prolongations  of  the  uncrossed  pyr- 
amidal tracts.  The  olivary  bodies,  oval  in  shape,  are  just  exter- 
nal to  the  anterior  pyramids  separated  from  them  by  a  groove. 
The  restiform  bodies  make  up  the  postero-lateral  portion  of  the 
medulla,  and  are  external  to  the  olivary  bodies.  They  contain 
fibers  from  the  columns  of  Burdach,  and  contribute  largely  to 
the  formation  of  the  inferior  peduncles  of  the  cerebellum.  The 
restiform  bodies,  diverging  as  they  ascend,  form  the  lateral 
boundaries  of  the  inferior  division  of  the  fourth  ventricle. 
Beneath  the  olivary  bodies,  and  between  the  anterior  pyramids 


252 


THE   NERVOUS    SYSTEM 


and  the  restiform  bodies,  are  the  lateral  fasciculi,  or  thefuniculi  of 
Rolando.  They  constitute  the  upward  prolongation  of  all  the 
antero-lateral  portion  of  the  cord  which  does  not  go  to  the  for- 
mation of  the  anterior  pyramids.  Their  chief  importance  is  in 
the  fact  that  they  contain  the  centers  for  respiration.  The 


FIG.  79. — Floor  of  the  4th  ventricle  and  the  connections  of  the  cerebellum. 

On  the  left  side  the  three  cerebellar  peduncles  are  cut  short;  on  the  right  the  con- 
nections of  the  superior  and  inferior  peduncles  have  been  preserved,  while  the  middle 
one  has  been  cut  short,  i,  median  groove  of  the  4th  ventricle  with  the  fasciculi 
teretes;  2,  the  striae  of  the  auditory  nerve  on  each  side  emerging  from  it;  3,  inferior 
peduncle;  4,  posterior  pyramid  and  clava,  with  the  calamus  scriptorius  above  it;  5, 
superior  peduncle;  6,  fillet  to  the  side  of  the  crura  cerebri;  8,  corpora  quadrigemma. 
(Landois.) 

posterior  pyramids  are  sometimes  called   the  funiculi  graciles. 
They  join  the  restiform  bodies  and  pass  to  the  cerebellum. 

The  fourth  ventricle  deserves  paricular  attention.  It  is  a 
cavity  on  the  posterior  aspect  of  the  pons  and  medulla  extending 
from  the  upper  limit  of  the  former  to  a  point  on  the  latter  oppo- 
site the  lower  border  of  the  olivary  body.  It  has  the  shape  of 
two  isosceles  triangles  placed  base  to  base.  The  apex  of  the 
inferior  triangle  is  at  the  calamus  scriptorious,  and  its  lateral 
boundaries  are  eht  diverging  restiform  bodies.  The  superior 


THE    MEDULLA    OBLONGATA  253 

peduncles  of  the  cerebellum  form  the  lateral  boundaries  of  the 
superior  triangle.  The  inferior  triangle  is  covered  by  the  cere- 
bellum; the  superior  by  the  valve  of  Vieussens,  which  stretches 
between  the  superior  peduncles.  This  ventricle  communicates 
above  with  the  third  ventricle  by  the  aqueduct  of  Sylvius,  or  the 
iter  a  tertio  ad  quartum  ventriculum;  below,  with  the  central  canal 
of  the  cord  and  with  the  subarachnoid  space.  The  floor  of  the 
ventricle  presents  a  longitudinal  median  fissure  and  numerous 
small  elevations  indicating  the  position  of  the  nuclei  of  origin  of 
certain  of  the  cranial  nerves. 

The  gray  matter  of  the  medulla  has  the  same  general  distri- 
bution as  that  in  the  cord,  but  is  by  no  means  so  regular  in  its 
disposition.  The  direction  of  the  white  fibers  is  not  so  uniform 
as  in  the  cord.  They  run  not  only  longitudinally,  but  trans- 
versely to  connect  the  lateral  halves,  and  in  other  directions  to 
connect  various  centers  situated  in  this  part  of  the  encephalon 
and  to  connect  the  medulla  with  other  parts  of  the  brain.  The 
following  is  the  relation  of  the  columns  of  the  cord  to  the  medulla: 

The  direct  and  crossed  pyramidal  tracts  pass  to  the  encephalon 
constituting,  in  the  medulla,  the  anterior  pyramids — the  direct, 
having  decussated  below,  occupying  here  the  outer  portion  of 
the  pyramid,  and  the  crossed  decussating  in  the  medulla,  and 
occupying  the  inner  portion  of  the  pyramid. 

Those  columns  concerned  in  reflex  action,  the  anterior  funda- 
mental fasciculi,  the  anterior  root  zones,  and  the  mixed  lateral 
tracts  do  not  continue  farther  upward  than  the  gray  matter  of 
the  medulla. 

The  columns  of  G oil  are  continuous  with  the  funiculi  graciles. 

The  columns  of  Burdach  and  the  direct  cerebellar  fasciculi  pass 
to  the  cerebellum  through  the  restiform  bodies  of  the  medulla. 

Functions. — The  functions  of  the  medulla  are  (i)  conduction, 
(2)  reflex  action,  (3)  to  furnish  centers  for  special  acts. 

i.  As  a  conductor  the  medulla  is  absolutely  necessary  as  a 
means  of  connection  between  the  brain  and  cord.  Sensory  im- 


254  THE   NERVOUS    SYSTEM 

pressions  to  and  motor  impulses  from  the  brain  must  all  pass 
through  by  this  route. 

As  a  reflex  nerve  center  the  medulla  also  resembles  the  cord, 
though  impressions  reflected  through  this  organ  are  frequently 
much  less  simple  than  those  reflected  through  the  cord.  Reflex 
action  in  the  medulla  is  dependent  on  (3),  to  be  noticed  now. 

3.  The  most  important  center  presiding  over  coordinated 
movements  is  that  for  respiration.  The  encephalon  may  be  cut 
away  down  as  far  as  the  medulla,  and  life  will  continue  for  a  cer- 
tain time.  It  is  al so  ti  ue  that  the  medulla  itself  may  be  gradually 
cut  away  from  above  downward  until  a  certain  point  is  reached, 
when  respiration  suddenly  ceases.  Likewise  the  spinal  cord  may 
be  cut  away  upward  till  this  point  is  reached,  when  the  same  re- 
sults will  follow.  This  is  the  true  respiratory  center,  and  is  situ- 
ated at  the  site  of  origin  of  the  vagi.  Its  destruction  is  followed 
by  an  immediate  suspension  of  respiration  and  consequent  death 
by  asphyxia,  though  there  is  no  manifestation  of  the  distress 
usually  accompanying  this  condition.  The  sense  of  want  of  air  is 
simply  lost.  There  is  one  of  these  centers  for  each  side,  but  they 
act  synchronously,  being  connected  by  commissural  fibers. 
Probably  the  usual  mode  of  stimulation  of  the  respiratory  center 
is  by  afferent  impressions,  but  it  may  also  be  stimulated  directly,  as 
by  deoxygenated  blood.  Mutilation  of  the  medulla,  on  account 
of  the  presence  of  this  center,  is  followed  by  the  nearest  approach 
to  instantaneous  death,  and  the  respiratory  center  has,  therefore, 
been  called  the  "vital  spot,"  though  death  from  any  cause  can- 
not be  instantaneous. 

Some  other  reflex  centers  are  for  deglutition,  sucking,  secretion 
of  saliva,  vomiting,  coughing,  sneezing,  dilatation  of  the  pupil,  se- 
cretion of  sweat,  secretion  of  glycogen,  etc.  Typical  of  these  is 
the  reflex  act  of  sneezing,  in  which  case  impressions  are  conveyed 
to  the  medulla  by  the  nasal  branches  of  the  fifth  nerve. 

Additional  centers  in  the  medulla  are  those  which  preside  over 
inhibition  and  acceleration  of  the  heart,  vaso-motor  centers  for  the 


THE   PONS    VAROLII  255 

vessel  walls,  and  centers  for  special  senses  like  hearing  and  taste. 
There  is  also  said  to  be  here  a  center  controlling  the  production 
of  heat  by  the  tissues. 

The  Pons  Varolii. 

Anatomy. — The  pons  is  situated  just  above  the  medulla  ob- 
longata  at  the  base  of  the  brain,  and  is  frequently  called  the 
great  commissure,  for  the  reason  that  it  contains  white  fibers  con- 
necting the  two  lateral  halves  of  the  cerebellum  and  the  differ- 
ent portions  of  the  cord  and  medulla  with  the  parts  of  the  brain 
above.  It  resembles  the  cord  in  having  its  white  matter  situated 
externally, 'while  within  its  substance  are  a  number  of  collections 
of  gray  matter.  The  longitudinal  fibers  are  continuations  upward 
of  fibers  from  the  olivary  bodies  and  the  anterior  pyramids  of  the 
medulla  and  also  of  parts  of  the  posterior  and  lateral  columns 
of  the  cord.  They  pass  through  the  crura  cerebri  to  the  brain. 

Functions. — The  anatomical  structure  and  situation  of  the 
pons  at  once  suggest  that  its  function  is  to  transmit  motor 
impulses  from  and  sensory  impressions  to  the  cerebrum. 

The  gray  centers,  however,  indicate  a  further  function  of  this 
organ.  It  is  found  that  the  removal  of  all  parts  of  the  enceph- 
alon  above  the  pons  does  not  deprive  an  animal  of  voluntary 
motion  and  general  sensibility.  It  will  be  seen  later  that  the 
integrity  of  the  cerebrum  is  essential  to  any  intellectual  opera- 
tion, and  manifestly,  under  the  conditions  mentioned,  there  can 
be  no  voluntary  motion  which  indicates  any  degree  of  intelligence; 
but  the  fact  remains  that  the  animal  can  perform  movements 
which  are  different  from  the  reflex  movements  depending  on  the 
presence  of  the  cord  when  all  other  parts  of  the  cerebro-spinal 
axis  have  been  removed.  The  pons  is  apparently  "an  organ 
capable  of  originating  impulses  giving  rise  to  voluntary  move- 
ments, when  the  cerebrum,  corpora  striata  and  optic  thalami 
have  been  removed,  and  it  probably  regulates  the  automatic 
voluntary  movements  of  station  and  progression."  (Flint.) 


256  THE   NERVOUS    SYSTEM 

Nor  can  it  be  doubted  that  an  animal  thus  mutilated  feels 
pain.  It  is  probable  that  the  sensory  impression  is  received  by 
some  of  the  gray  centers  in  the  pons  itself,  but  not  being  con- 
veyed to  the  cerebrum,  is  not  remembered. 

The  Crura  Cerebri,  Corpora  Striata,   Optic   Thalami,   Internal 
Capsule  and  Corpora  Quadrigemina. 

It  will  be  well  before  discussing  the  cerebrum  to  consider 
briefly  other  collections  of  gray  and  white  matter  in  the  neigh- 
borhood of  the  upper  part  of  the  pons. 

The  crura  cerebri,  passing  upward  from  the  anterior  part  of 
the  pons,  diverge  to  run  apparently  underneath  the  corpora 
striata  and  optic  thalami  in  the  direction  of  the  cerebral  hemis- 
pheres. They  are  about  j  inch  long  and  slightly  broader  above 
than  below.  The  main  bulk  of  each  crus  consists  of  white  fibers, 
but  a  collection  of  gray  matter  (locus  niger)  divides  the  band  into 
a  lower  or  superficial  section,  called  the  crusta,  and  an  upper  or 
deep  section,  called  the  tegmentum.  There  is  also  some  gray 
matter  in  the  tegmentum  proper.  The  fibers  of  the  tegmentum 
are  supposed  to  convey  afferent  impressions  chiefly,  and  end  for 
the  most  part  in  the  optic  thalamus,  though  some  are  continued  to 
the  cerebrum  through  the  internal  capsule.  The  fibers  of  the 
crusta  are  supposed  to  convey  efferent  impulses,  and  pass  to  the 
corpus  striatum  and  the  cerebrum. 

It  is  evident  that  the  function  of  the  crura  is  mainly  to  con- 
duct messages  to  and  from  the  parts  above.  It  is  said  that  the 
locus  niger  is  concerned  in  coordination  of  the  movements  of 
the  eye-ball  and  iris. 

The  Corpora  Striata,  Optic  Thalami  and  Internal  Capsule 
are  closely  related  and  are  best  considered  together. 

Each  corpus  striatum  is  pear-shaped  with  its  large  end  forward 
and  near  the  median  line;  the  posterior  small  extremities  are 
divergent  from  each  other  and  embrace  the  two  optic  thalami. 
Externally  they  are  white;  internally  white  and  gray  elements 


THE    OPTIC   THALAMI 


257 


are  mixed.  Each  is  separated  by  the  anterior  limb  of  the  internal 
capsule  into  two  divisions,  external  and  internal,  known  respec- 
tively as  the  lenticular  and  caudate  nuclei.  (See  Fig.  80.) 


FIG.  80. — Human  brain,  with  the  hemispheres,  removed  by  a   horizontal 

incision  on  the  right  side. 

4,  trochlear;  8,  acoustic  nerve;  6,  origin  of  the  abducens;  F,  A,  L,  position  of  the 
pyramidal  (motor)  fibers  for  the  face,  arm  and  leg;  S,  sensory  fibers.     (Landois.) 

The  optic  thalami,  one  on  either  side,  have  an  oval  shape  and 
rest  upon  the  crura  cerebri  between  the  posterior  extremities  of 

17 


258  THE   NERVOUS   SYSTEM 

the  two  corpora  striata.  Most  of  their  external  surface  is  white ; 
internally  each  possesses  six  gray  nuclei. 

Separating  the  two  nuclei  of  the  corpus  striatum  anteriorly, 
and  the  lenticular  nucleus  from  the  optic  thalamus  posteriorly, 
is  a  band  of  white  fibers  known  as  the  internal  capsule.  The 
part  between  the  two  nuclei  is  the  anterior  limb ;  that  between  the 
lenticular  nucleus  and  the  optic  thalamus  is  the  posterior  limb. 
These  limbs,  joining  at  an  obtuse  angle,  constitute  a  bend  in  the 
internal  capsule  which  is  called  the  genu,  or  knee.  The  fibers 
of  the  capsule  pass  to  the  frontal,  parietal  and  occipital  lobes  of 
the  cortex,  and  in  their  course  to  these  parts  they  diverge  to 
form  the  corona  radiata. 

External  to  the  lenticular  nucleus  is  a  band  of  white  fibers 
known  as  the  external  capsule.  In  it  is  a  longitudinal  mass  of 
gray  matter,  the  claustrum.  Fig.  76  shows  the  relations  of  these 
parts. 

Functions.— The  exact  function  of  the  corpora  striata  is  a 
matter  of  some  doubt.  They  have  been  considered  the  great 
motor  ganglia  of  the  base  of  the  brain;  but,  although  lesions 
here  are  followed  by  paralysis  on  the  opposite  side  of  the  body, 
it  is  held  that  this  phenomenon  is  due  to  the  proximity  of  the 
internal  capsule.  The  further  fact  that  irritation  of  this  organ 
is  followed  by  muscular  contractions  does  not  prove  that  it 
ordinarily  generates  motor  force,  for  many  of  the  fibers  from  the 
motor  cortical  zone  pass  to  or  through  the  corpus  striatum. 
This  may  be  only  a  relay  station,  and  the  corpus  may  be  quite 
subsidiary.  It  undoubtedly,  however,  is  connected  with  motion 
in  some  way. 

The  precise  function  of  the  optic  thalami  is  equally  obscure. 
The  relation  of  these  organs  to  the  tegmenta  would  suggest  that 
they  have  something  to  do  with  the  sensory  fibers  on  their  way 
to  the  cortex.  It  cannot  be  denied  that  they  are  concerned  in 
sensation,  since  their  removal  is  followed  by  crossed  anesthesia. 
They  may  likewise  be  relay  stations.  Each  sends  fibers  to  the 


THE   BASAL   GANGLIA  259 

cerebellum  and  contains  one  of  the  nuclei  of  origin  of  the  optic 
nerve. 

Regarding  the  function  of  the  internal  capsule  it  may  be  said 
that  its  fibers  are  in  main  part  prolongations  from  the  crusta  and 
from  the  gray  matter  of  the  corpora  striata;  fibers  also  pass  up- 
ward through  it  from  the  tegmentum  and  the  optic  thalamus.  As 
a  matter  of  fact,  most  of  the  fibers  of  the  crura  go  directly  into 
the  corpora  striata  (motor)  and  the  optic  thalami  (sensory),  but 
some  pass  directly  upward  through  the  capsule.  It  is  to  be 
noted,  however,  that  the  capsule  does  not  consist  of  these  last 
named  fibers  alone,  but  of  fibers  from  the  corpora  striata  and 
optic  thalami  as  well.  Observations  show  that  pathological 
lesions  affecting  the  anterior  two-thirds  of  the  posterior  division 
of  the  internal  capsule  are  followed  by  paralysis  of  motion;  that 
lesions  affecting  only  the  posterior  one-third  of  the  posterior  di- 
vision are  followed  by  anesthesia;  and  that  lesions  affecting 
the  entire  posterior  limb  are  followed  by  both  paralysis  and  an- 
esthesia— these  phenomena  always  manifesting  themselves  on  the 
side  opposite  the  lesion  only.  This  leads  to  a  definite  conclusion  ; 
viz.,  that  efferent  fibers  occupy  the  anterior  two-thirds  and  afferent 
fibers  the  posterior  one-third  of  the  posterior  limb  of  the  capsule. 

Nothing  conclusive  can  be  said  about  the  function  of  the  ex- 
ternal capsule  or  of  the  claustrum. 

The  Corpora  Quadrigemina,  two  on  each  side,  are  promi- 
nences on  the  dorsal  surface  of  the  pons  and  crura  above  the  aque- 
duct of  Sylvius.  They  contain  white  and  gray  matter.  The  pos- 
terior tubercle's  are  connected  with  the  eighth  nerve,  the  sen- 
sory tract,  the  temporal  region  of  the  brain,  and  the  lateral  cor- 
pora geniculata.  The  anterior  tubercles  are  connected  with  the 
optic  nerve,  with  the  occipital  region,  and  with  the  median  cor- 
pora geniculata. 

The  function  of  the  anterior  of  these  bodies  is  mainly  con- 
nected with  the  eye;  the  posterior  are  associated  with  the  sense 
of  hearing. 


260  THE    NERVOUS    SYSTEM 

The  Cerebrum. 

The  great  size  of  the  cerebral  hemispheres  in  man  obscures 
the  fact  that  the  different  parts  of  the  brain  are  disposed  in  a 
linear  series;  these,  from  before  backward,  are,  the  olfactory  lobes, 
cerebral  hemispheres,  optic  thalami,  corpora  quadrigemina, 
cerebellum,  medulla  oblongata.  This  arrangement  exists  in 
the  human  fetus,  and  persists  throughout  life  in  some  of  the 
lower  animals. 

Anatomy. — The  substance  of  each  hemisphere  is  divided  by 
fissures  into  five  lobes — (a)  frontal,  (b)  parietal,  (c)  occipital,  (d) 
temporo-sphenoidal  and  (e)  central.  The  main  fissures  are  four 
in  number — (i)  The  fissures  of  Sylvius  running  from  the  front 
and  under  part  of  the  brain  backward,  outward  and  upward;  (2) 
the  fissures  of  Rolando  running  from  the  median  line  near  the 
center  of  the  longitudinal  fissure  forward,  outward  and  down- 
ward; (3)  the  parieto-occipital  fissure,  little  of  which  is  evident 
upon  the  surface  of  the  brain,  but  which  appears  on  longitudinal 
section  separating  the  occipital  and  parietal  lobes;  (4)  the  calloso- 
marginal  fissure,  also  evident  only  on  the  internal  aspect  of  the 
hemisphere,  parallel  with  and  above  the  corpus  callosum.  (Figs. 
81,  82.) 

(a)  The  frontal  lobe  is  bounded  internally  by  the  longitudinal 
fissure,  posteriorly  by  the  fissure  of  Rolando  and  below  by  the 
fissure  of  Sylvius.     On  its  surface  are  seen  three  convolutions, 
approximately  parallel,  called  the  superior,  middle  and  inferior 
frontal  convolution,  and  occupying  positions  which  their  names 
indicate.     In  addition  the  posterior  portion  of  this  lobe  is  occu- 
pied by  the  ascending  frontal,  or  the  anterior  central  convolution, 
lying  just  in  front  of  the  Rolandic  fissure. 

(b)  The  parietal  lobe  is  bounded  anteriorly  by  the  fissure  of 
Rolando,  internally  by  the  longitudinal  fissure,  posteriorly  by 
the  parieto-occipital  fissure  and  below  by  the  fissure  of  Sylvius. 
Just  behind  the  fissure  of  Rolando  is  the  ascending  parietal,  or 


THE    CEREBRUM 


261 


posterior  central  convolution,  above,  this  is  continuous  with  the 
upper  parietal  convolution,  below  which  is  the  inferior  parietal 
lobule  separated  from  the  preceding  by  the  intra-parietal  sulcus. 
This  inferior  parietal  lobule  winds  around  the  posterior  part  of 


FIG.  81. — Left  side  of  the  human  brain  (diagrammatic). 

F,  frontal;  P,  parietal;  O,  occipital;  T,  temporo-sphenoidal  lobe;  S,  fissure  of 
Sylvius;  S',  horizontal;  S",  ascending  ramus  of  S;  c,  sulcus  centralis,  or  fissure  of 
Rolando;  A,  ascending  frontal,  and  B,  ascending  parietal  convolution;  Fi,  suoerior, 
F2,  middle,  and  Fs,  inferior  frontal  convolutions;/!,  suoerior,  and/2,  inferior,  frontal 
fissures;/?,  sulcus  precentralis ;  P,  superior  parietal  lobule;  P2,  inferior  parietal  lobule- 
consisting  of  P2,  supra-marginal  gyrus,  and  P2',  angular  gyrus;  ip,  sulcus  interpariet' 
alis;  cm,  termination  of  calloso-marginal  fissure;  O,  first;  Os,  second;  Os,  third 
occipital  convolutions;  po,  parietal-occipital  fissure;  o,  transverse  occipital  fissure; 
02,  inferior  longitudinal  occipital  fissure;  Ti,  first;  T2,  sec9nd;  Ta,  third,  temporo- 
sphenoidal  convolutions;  ti,  first;  tz,  second,  temporo-sphenoidal  fissures.  (Landois.) 


262 


THE   NERVOUS    SYSTEM 


the  fissure  of  Sylvius,  and  is  divided  into  the  supra-marginal 
convolution,  embracing  the  short  arm  of  this  fissure,  and  the 
angular  convolution  connecting  below  with  the  temporal  lobe. 
(c)  The  occipital  lobe  is  situated  posteriorly  below  the  parieto- 


FIG.  82. — Median  aspect  of  the  right  hemisphere. 

CC,  corpus  callosum  divided  longitudinally;  Gf,  gyrus  fornicatus;  H,  gyrus  hip- 
pocampi; h,  sulcus  hippocampi;  U,  uncinate  gyrus;  cm,  calloso-marginal  fissure;  F, 
first  frontal  convolution;  c,  terminal  portion  of  fissure  of  Rolando;  A,  ascending 
frontal;  B,  ascending  parietal  convolution  and  paracentral  lobule;  Pi',  parecuneus 
or  quadrate  lobule;  Oz,  cuneus;  Po,  parieto-occipital  fissure;  o',  transverse  occipital 
fissure;  oc,  calcarine  fissure;  pc',  superior;  oc",  inferior  ramus  of  the  same;  G,  gyrus 
descendens;  T<,  gyrus  occipito-temporalis  lateralis  (lobulus  fusiformis);  Ta,  gyrus 
occipito-temporalis  medialis  (lobulus  lingualis).  (Landois.) 

occipital  fissure  and  external  to  the  median  fissure.  It  presents 
three  convolutions,  the  superior,  middle  and  inferior. 

(d)  The  temporo-sphenoidal  lobe  is  below  the  fissure  of  Sylvius 
in  front  of  the  occipital  lobe.     It  likewise  presents  superior, 
middle  and  inferior  convolutions. 

(e)  The  central  lobe,  or  island  of  Reil,  presents  the  gyrus  forni- 
catus, a  convolution  curving  around  the  corpus  callosum;  the 


THE    CEREBRUM  263 

marginal  convolutions  beyond  the  calloso-marginal  fissure  from 
the  preceding  and  between  it  and  the  edge  of  the  longitudinal 
fissure;  the  continuation  of  the  parieto-occipital  fissure  running 
downward  and  forward  to  meet  the  calcarine  fissure,  between 
which  is  the  cuneus;  the  internal  aspect  of  the  temporal  lobe, 
the  uncinate  gyrus. 

Structure. — The  cerebral  hemispheres  are  composed  of  white 
and  gray  matter,  but  here  the  gray  matter  is  situated  externally. 
To  increase  its  amount,  with  economy  of  space,  the  gray  matter 
is  thrown  into  many  convolutions,  to  some  of  which  reference  has 
been  made.  The  sulci  separating  these  convolutions  have  a 
depth  in  the  average  human  brain  of  about  one  inch.  The  thick- 
ness of  the  gray  matter  of  the  cortex  varies  from  -^  to  J  in., 
being  thinnest  in  the  occipital  and  thickest  in  the  front  parietal 
region. 

The  cells  found  in  the  superficial  and  deep  portions  of  the 
gray  matter  are  not  uniform  in  size  or  shape.  In  a  general  way 
it  may  be  said  that  they  increase  in  size  as  the  surface  is  left, 
but  in  addition  to  the  comparatively  large  cells  in  the  deep  parts 
there  are  also  numbers  of  small  ones.  Passing  in  the  same  direc- 
tion there  are  found  in  succession  small  pyramidal,  larger  pyram- 
idal, and  irregular  branching  cells. 

Fibers  from  the  Cerebrum.— Fibers  pass  from  each  cerebral 
hemisphere  to  (a)  the  spinal  cord,  (b)  the  cerebellum,  (c)  the 
opposite  cerebral  hemisphere,  and  (d)  different  parts  of  the  same 
hemisphere. 

(a)  Fibers  converge  from  the  anterior  and  middle  (particularly 
the  latter)  parts  of  the  cortex  to  pass  by  the  corona  radiata  to 
the  corpora  striata,  from  which  fibers  are  continued  to  the  crusta, 
pons,  pyramids  of  the  medulla  and  pyramidal  tracts  of  the  cord; 
most  of  these  pass  down  through  the  internal  capsule  to  reach 
the  corpora  striata.  From  the  same  regions  also  some  fibers 
pass  directly  through  the  internal  capsule,  without  connection 
with  the  corpora  striata,  to  be  actually  continuous  themselves 


264  THE   NERVOUS    SYSTEM 

with  fibers  which,  following  the  same  course  downward,  are  found 
in  the  pyramidal  tracts  of  the  cord.  All  fibers  passing  from 
these  cortical  areas  mentioned  through  the  internal  capsule 
occupy  the  anterior  two-thirds  of  the  posterior  division  of  that 


FIG.  83. — Scheme  of  the  projection  fibers  within  the  brain.     (Starr.) 

Lateral  view  of  the  internal  capsule;  A,  tract  from  the  frontal  gyri  to  the  pons 
nuclei,  and  so  to  the  cerebellum;  B,  motor  tract;  C,  sensory  tract  for  touch  (sepa- 
rated from  B  for  the  sake  of  clearness  in  the  scheme);  D,  visual  tract;  E,  auditory 
tract;  F,  G,  H,  superior,  middle,  and  inferior  cerebellar  peduncles;  J,  fibers  between 
the  auditory  nucleus  and  the  inferior  quadrigeminal  body;  K,  motor  decussation  in 
the  bulb;  At,  fourth  ventricle.  The  numerals  refer  to  the  cranial  nerves.  The  sen- 
sory radiations  are  seen  to  be  massed  toward  the  occipital  end  of  the  hemisphere. 
(Am.  Text-book.) 

tract.  Furthermore,  fibers  from  the  posterior  cortical  area  pass 
through  the  posterior  one-third  of  the  posterior  division  of  the 
internal  capsule  to  the  optic  thalamus,  from  which  fibers  pass 
through  the  tegmentum  to  the  pons  and  medulla  and  are  continu- 
ous with  fibers  from  the  sensory  tracts  of  the  cord.  The  decus- 
sation of  all  these  fibers  has  been  mentioned. 

Fig.  84  taken  in  conjunction  with  Fig.  77  illustrates  the  most 
recent  ideas  of  the  motor  and  sensory  connections  between  brain 
and  cord  and  the  motor  and  sensory  paths  in  the  cord. 


THE   CEREBRUM 


265 


A.O.N 


FIG.  84. — Scheme  of  relationship  of  cells  and  fibers  of  brain  and    cord. 

(Kirkes.) 

Pyr,  cell  of  Rolandic  area;  Ax,  its  axis  cylinder  crossing  the  middle  line  AB,  to 
enter  one  of  the  pyramidal  tracts;  the  collateral  Call  goes  to  the  cortex  of  the  oppo- 
site hemisphere,  while  another,  str,  enters  the  corpus  striatum.  The  axis  cylinder 
arborizes  around  an  anterior  horn  cell,  whence  a  motor  fiber  goes  to  the  muscle. 

The  axis  cylinder  from  the  spinal  ganglion  cell  is  represented  as  bifurcating  and 
sending  one  branch  to  the  periphery  and  one  to  the  cord;  the  latter  itself  bifurcates, 
the  lower  division  ending  as  shown  better  in  Fig.  77.  N.G,  cell  in  posterior  cornu  of 
the  cord  or  posterior  column  of  the  bulb.  The  distance  of  this  cell  from  the  point  of 
entrance  of  the  axis  cylinder  into  the  cord  may  be  great  or  small.  Note  the  collat- 
erals from  it  in  Fig.  77.  I.A,  decussating  fiber  ending  at  cell  in  optic  thalamus,  O.T, 
from  which  a  fiber  passes  to  the  cortex.  A  collateral  is  shown  passing  from  the 
ascending  sensory  fiber  to  a  cell  of  Clarke's  column,  whence  a  fiber  passes  to  a  cell,  P, 
of  the  cerebellum. 


266  THE   NERVOUS   SYSTEM 

(b)  Fibers  from  the  anterior  portion  of  the  frontal  lobe  pass 
through  the  anterior  limb  of  the  internal  capsule  and  seem  to 
end  in  the  gray  matter  of  the  pons  and  there  to  communicate  with 
the  cerebellum  through  the  middle  peduncles.  Fibers  also  pass 
from  the  temporo-sphenoidal  lobes  and  from  the  caudate  nuclei 
of  the  corpora  striata  to  the  cerebellum  on  the  opposite  side. 
The  connection  is  crossed  in  all  these  cases. 


FIG.  85. — Diagram  of  the  motor  areas  on  the  outer  surface  of  a  monkey's 
brain.     (Landois  after  Horsley  and  Schafer.) 


(c)  Transverse  fibers  in  the  corpus  callosum  connect  all  parts 
of  the  two  lateral  hemispheres.     Besides  these  commissural  fibers 
there  are  those  of  the  anterior  and  posterior  white  commissures. 
Fibers  in  the  anterior  connect  the  tempero-sphenoidal  lobes  and 
probably  the  corpora  striata  with  each  other;  fibers  in  the  pos- 
terior connect  the  temporo-sphenoidal  lobes  with  the  optic  thai- 
ami  of  the  opposite  side. 

(d)  The  arcuate  fibers  connect  different  convolutions  of  the 
same  lobe  and  the  convolutions  of  different  lobes  with  each  other. 
Some  of  these  are  in  the  fornix,  in  the  corpus  callosum,  and  in 
other  parts,  as  well  as  running  along  the  concave  surface  of  the 
cortex. 


THE   CEREBRUM 


267 


FIG.  86. — Side  view  of  the   brain  of  man,  with  the  areas  of  the  cerebral 
convolutions  according  to  Ferrier.     (Brubaker.) 

The  figures  are  constructed  by  marking  on  the  brain  of  man,  in  their  respective 
situations,  the  areas  of  the  brain  of  the  monkey  as  determined  by  experiment,  and 
the  description  of  the  effects  of  stimulating  the  various  areas  refers  to  the  brain  of 
the  monkey. 

i,  advance  of  the  opposite  hind  limb,  as  in  walking;  2,  3,  4,  complex  movements 
of  the  opposite  leg  and  arm,  and  of  the  trunk,  as  in  swimming;  a,  b,  c,  d,  individual 
and  combined  movements  of  the  fingers  and  wrist  of  the  opposite  hand.  Prehensile 
movements.  5 ,  extension  forward  of  the  opposite  arm  and  hand;  supination  and 
flexion  of  the  opposite  forearm;  7,  retraction  and  elevation  of  the  opposite  angle  of 
the  mouth  by  means  of  the  zygomatic  muscle;  8,  elevation  of  the  alae  nasi  and  upper 
lip,  with  depression  of  the  lower  lip  on  the  opposite  side;  9,  10,  opening  of  the  mouth, 
with  (9)  protrusion  and  (10)  retraction  of  the  tongue;  region  of  aphasia,  bilateral 
action;  n,  retraction  of  the  opposite  angle  of  the  mouth,  the  head  turned  slightly  to 
one  side;  12,  the  eyes  open  widely,  the  pupils  dilate,  and  the  head  and  eyes  turn 
toward  the  opposite  side;  13,  13',  the  eyes  move  toward  the  opposite  side,  with  an 
upward  (13)  or  downward  (13')  deviation;  the  pupils  are  generally  contracted ;  14, 
pricking  of  the  opposite  ear,  the  head  and  eyes  turn  to  the  opposite  side,  and  the 
pupils  dilate  widely. 


268  THE   NERVOUS    SYSTEM 

Cerebral  Localization. — There  are  certain  cortical  areas 
which  have  certain  fixed  functions.  There  are  certainly  such 
areas  for  motion  and  for  the  reception  of  impressions  conveyed  by 
the  nerves  of  special  sense;  areas  for  the  reception  of  impressions 
conveyed  by  the  nerves  of  general  sensation  have  not  been  defin- 
itely determined. 

Motor  Centers. — Electrical  stimulation  of  the  convex  surface 
of  the  cerebrum  shows  that  the  anterior  part  is  motor  and  the 
posterior  part  non-motor;  that  stimulation  of  the  motor  por- 
tion produces  muscular  contractions  on  the  opposite  side  of  the 
body,  that  stimulation  in  the  same  spot  is  always  followed  by 
the  same  contractions;  and  that  when  the  current  is  quite  weak 
the  contractions  are  limited  to  distinct  muscles  or  sets  of  muscles. 
It  may  be  further  said  that  while  experiments  establishing  these 
facts  have  been  largely  limited  to  inferior  animals,  the  deductions 
have  been  made  applicable  to  man  by  pathological  observations 
and  by  the  fact  that  in  different  animals  stimulation  of  anatom- 
ically corresponding  parts  is  followed  by  corresponding  results. 
Destruction  of  motor  areas  is  followed  by  descending  secondary 
degeneration  of  fibers  through  the  corona  radiata,  internal  cap- 
sule, crura  cerebri  (crusta),  anterior  pyramids  of  the  medulla  and 
the  pyramidal  tracts  of  the  cord;  the  resulting  paralysis  is  on  the 
side  opposite  the  lesion. 

The  motor  cortical  zone,  so  far  as  can  now  be  said,  corresponds 
to  the  ascending  frontal  and  parietal  convolutions  on  either  side 
of  the  fissure  of  Rolando,  to  the  paracentral  lobule,  and  possibly 
to  a  small  area  in  front  of  the  ascending  frontal  convolution. 
From  above  downward,  on  either  side  of  the  Rolandic  fissure 
are  areas  presiding  over  the  movements  of  the  leg,  arm  and 
face. 

More  specific  information  as  regards  areas  controlling  various 
movements  may  be  obtained  by  reference  to  Fig  86. 

Various  kinds  of  monoplegia  (crossed)  are  caused  by  lesions, 
as  hemorrhage,  in  localized  parts  of  the  motor  area;  there  may 


THE   CEREBRUM  269 

be  facial,  brachial,  crural,  brachio-facial  monoplegia,  etc.  There 
can  be  no  doubt  that  from  the  motor  cortical  zone  pass  the  fibers 
which  constitute  the  pyramidal  tracts  of  the  cord. 

Sensory  Centers. — Centers  for  the  reception  of  impressions 
giving  rise  to  general  sensation  may  exist.  Fibers  from  the  tem- 
poro-sphenoidal  and  occipital  lobes  pass  through  the  posterior 
third  of  the  posterior  division  of  the  internal  capsule,  and  it  may, 
therefore,  be  assumed  that  these  parts  of  the  cerebrum  are  con- 
nected with  general  sensation. 

Special  Centers. — Besides  these  areas  for  motion  and  general 
sensation,  special  centers  certainly  exist. 

The  Optic  Center  is  in  the  occipital  lobe,  probably  in  the  cuneus. 
Removal  of  the  right  occipital  lobe  is  followed  by  lefthemiopia 
and  vice  versa;  removal  of  both  causes  total  blindness. 

The  Olfactory  Center  is  probably  on  the  inner  surface  of  the 
anterior  extremity  of  the  uncinate  gyms  (inner  extremity  of  the 
temporal  lobe) . 

The  Gustatory  Center  is  supposed  to  be  in  the  temporal  lobe 
very  near  the  preceding. 

The  Auditory  Center  is  located  in  the  superior  and  middle 
convolutions  of  the  temporo-sphenoidal  lobe. 

The  Center  for  Cutaneous  Sensations  cannot  be  strictly  lim- 
ited, though  it  is  said  to  correspond  with  the  motor  area. 

The  Center  for  Muscular  Sensations  is  thought  to  be  in  the 
lower  parietal  region. 

The  Speech  Center. — One  may  not  be  able  to  speak  because 
he  cannot  control  the  muscles  usually  involved  in  such  an  act, 
or  because  he  has  no  comprehension  of  the  meaning  of  words, 
or  because  he  is  incapable  of  forming  the  idea  which  links  the 
reception  of  the  impression  and  the  muscular  act.  Aphasia  is 
the  term  generally  applied  to  inability  to  express  one's  self  by 
language.  It  is  to  be  distinguished,  however,  from  aphonia, 
which  is  simply  a  loss  of  voice.  Ataxic  aphasia  is  an  inability 
to  express  ideas  only  by  reason  of  muscular  incoordination;  a 


270  THE    NERVOUS    SYSTEM 

person  so  affected  may  use  words,  but  he  cannot  tell  what  sounds 
he  is  going  to  utter;  his  ability  to  receive  ideas  is  unimpaired, 
and  he  can  express  his  own  ideas  in  writing.  When  there  is 
inability  to  express  ideas  in  writing,  because  of  muscular  inco- 
ordination,  a  condition  of  a  graphic  aphasia  is  said  to  exist.  There 
are  also  cases  in  which  a  person  cannot  comprehend  ideas  ex- 
pressed in  language  and  cannot  express  himself  by  either  speak- 
ing or  writing;  this  is  known  as  amnesic  aphasia.  It  is  not 
impossible  that  in  some  instances  ideas  may  be  received  and 
there  still  be  an  inability  to  express  one's  self  in  any  way.  It  is 
noted  that  when  the  hemiplegia  accompanying  the  aphasia  is 
marked  the  form  is  usually  ataxic;  when  there  is  no  hemiplegia 
the  aphasia  is  usually  amnesic. 

The  part  of  the  brain  presiding  over  speech  is  in  the  left  third 
frontal  convolution  near  the  island  of  Reil.  In  left-handed  per- 
sons its  usual  situation  is  almost  certainly  at  a  corresponding 
point  on  the  right  side.  Why  the  center  is  unilateral  has  not 
been  explained.  It  may  be  that  it  was  originally  bilateral,  and 
the  growth  of  the  right  has  been  stopped  by  the  superior  develop- 
ment of  the  left  side  of  the  brain.  It  is  at  least  noticed  that  the 
right  instead  of  the  left  side  of  the  brain  is  heavier  in  left-handed 
persons.  Fibers  from  this  center  (Broca's  convolution)  pass 
through  the  anterior  part  of  the  posterior  division  of  the  internal 
capsule  to  reach  the  left  crus,  leaving  which  they  enter  the  pons 
to  decussate  and  go  to  the  right  side  of  the  medulla. 

Functions  of  the  Cerebrum. — The  superior  development  of 
the  intellect  in  man  is  the  most  predominant  characteristic  dis- 
tinguishing him  from  the  lower  animals.  That  many  such  ani- 
mals are  possessed  of  a  certain  degree  of  intelligence  is  not  usu- 
ally denied;  and  the  nature  of  their  mental  operations,  though 
they  are  insignificant  as  compared  with  man's,  may  be  admitted 
as  identical  with  his.  The  most  striking  difference  in  the  nervous 
system  of  man  as  compared  with  that  of  inferior  animals  is 
the  large  size  of  the  cerebrum  in  the  former.  This  is  not  sur- 


THE    CEREBRUM  271 

prising  when  it  is  admitted  that  in  the  substance  of  this  part  of 
the  encephalon  is  the  seat  of  those  faculties  which  manifest 
themselves  in  mental  operations. 

The  seat  of  the  changes,  if  they  be  changes,  which  result  in 
mental  operations  is  supposed  to  be  in  the  frontal  lobes;  these 
are  insensible  and  inexcitable,  but  severe  injury  to  them,  as  by 
hemorrhage,  is  followed  by  a  cessation  of  mental  activity;  con- 
genital defects  also  cause  a  corresponding  decrease  in  the  mental 
caliber. 

From  what  has  been  said  it  is  evident  that  the  cerebral  hemis- 
pheres are  capable  of  generating  motor  impulses  and  receiving  im- 
pressions general  and  special;  but  predominating  in  importance 
over  these  functions  is  the  fact  that  the  gray  substance  of  the 
cerebrum  is  essential  to  the  exercise  of  the  intellect — even  to  the 
existence  of  that  indefinite  something  called  the  mind. 

It  is  by  the  cerebrum  that  we  perceive  and  retain  impressions, 
that  we  understand,  imagine,  reflect,  reason  and  judge,  and  thus 
concoct  and  issue  the  mandates  of  our  will.  It  is  the  link  which 
connects  our  impressions  and  our  purposeful  actions. 

In  animals  upon  which  experiments  have  been  made  it  is 
found  that  life  may  persist  for  'a  time  after  the  removal  of  the 
hemispheres,  and  that,  outside  of  the  cessation  of  mental  activity, 
the  results  are  not  so  marked  as  one  would  on  first  thought  sup- 
pose. Stupor  and  absence  of  the  ordinary  instinctive  acts  (as 
corresponding  in  a  way  with  acts  of  the  will  in  man)  are  noted, 
but  voluntary  motion  and  general  sensibility  are  not  destroyed,  and 
may  be  but  little  interfered  with.  Of  course  there  is  no  volun- 
tary motion  in  the  sense  of  carrying  out  the  behests  of  the  will, 
for  the  organ  of  the  will  is  destroyed;  nor  is  there  any  record  of 
painful  impressions,  for  the  organ  of  memory  is  absent.  But  the 
animal  can  pertorm  various  consecutive  and  coordinate  move- 
ments, such  as  walking,  swimming,  etc.  For  example,  a  pigeon 
thus  mutilated  will  fly  when  thrown  irtfo  the  air.  This  does  not 
argue  any  mental  operation.  A  person  does  not  ordinarily 


272  THE   NERVOUS    SYSTEM 

apply  his  mind  to  the  act  of  walking  or  standing;  his  mental 
faculties  may  be  as  completely  engaged  with  the  deepest  thoughts 
of  psychology,  literature,  medicine  or  other  subjects  while  walk- 
ing as  at  any  other  time.  True,  he  probably  started  with  some 
fixed  purpose  to  go  in  some  particular  direction  to  some  definite 
place,  but  the  act  of  progression  does  not  per  se  require  fixed  at- 
tention on  his  part.  So  in  the  case  of  the  pigeon;  it  does  not, 
make  up  its  mind  to  fly  at  all;  and  it  will  not  fly  without  being 
thrown  into  the  air,  or  the  application  of  some  other  similar 
stimulus;  nor  does  it  fly  in  any  particular  direction,  or  to  any 
particular  place.  It  is  reduced  to  the  condition  of  a  "  mechan- 
ism without  spontaneity."  It  can  perform  voluntary  move- 
ments but  cannot  originate  them  without  external  inter- 
vention. 

Animals  which  have  been  subjected  to  the  operation  men- 
tioned undoubtedly  feel  pain.  They  move  away  or  cry  out  on 
being  burned,  for  example.  The  coordination  of  their  move- 
ments and  the  cries  contrast  with  the  phenomena  (reflex)  fol- 
lowing such  stimulation  when  only  the  cord  is  left.  It  was  noted 
above  that  impressions  in  these  cases  are  probably  received  by 
the  gray  matter  of  the  pons  and  not  recorded. 

The  special  senses  of  sight  and  hearing  remain  after  the  re- 
moval of  the  cerebrum.  The  same  is  probably  true  of  taste  and 
smell.  • 

It  would  seem  that  the  cerebrum  is  a  kind  of  storehouse  in 
which  are  kept  all  the  materials  necessary  for  the  performance 
of  all  kinds  of  pre-determined  acts,  whether  they  manifest  them- 
selves in  speech,  or  thought,  or  muscular  action.  What  excites 
these  materials  to  activity — i.  e.,  what  excites  a  voulntary  act — 
is  not  clear.  We  know  certain  things  will  usually  excite  a  certain 
train  of  thought,  or  cause  us  to  will  to  do  or  say  certain  things. 
Such  phenomena  are  akin  to,  if  not  identical  with,  reflex  action. 
These  manifestations  of  our  voluntary  power  are  due  to  impres- 
sions conveyed  by  afferent  fibers  to  the  cortex;  indeed  it  may 


THE    CEREBRUM  273 

be  that  every  afferent  fiber  in  the  system  exerts  an  influence 
thus  indirectly  upon  the  organ  of  the  will,  and  the  impressions 
conveyed  by  them  are  reflected  in  one's  character  and  life. 
But  it  cannot  be  said  that  all  voluntary  activity  is  thus  of  a 
reflected  nature;  there  is  some  cause  other  than  the  reception  of 
afferent  impressions  which  sets  the  will  in  operation. 

Connection  Between  the  Brain  and  Intelligence. — It  is 
claimed  that  a  single  hemisphere  is  capable  of  performing  all  the 
ordinary  intellectual  acts  as  well  as  both;  and  atrophy,  or  de- 
struction otherwise,  of  one  hemisphere  has  frequently  been  no- 
ticed to  entail  no  mental  defect.  But  whether  the  mind  under 
such  conditions  would  be  equal  to  the  highest  intellectual  attain- 
ments is  doubtful.  It  would  seem  that  in  health  the  brain  unites 
the  impressions  received  by  the  two  sides  (as,  e.  g.,  through  the 
optic  nerves),  and  the  resulting  idea  is  a  single  one;  that  is  to  say 
a  person  does  not  have  two  opposing  ideas  about  the  same  thing 
the  same  time;  the  two  hemispheres  seem  to  agree. 

In  a  general  way,  it  may  be  stated  that  the  degree  of  intelli- 
gence corresponds  to  the  weight  of  the  brain,  though  to  this 
rule  there  are  many  exceptions.  It  may  be  more  properly  said 
that  the  development  of  the  intellectual  faculties  is  greater  as 
the  area  of  gray  matter  is  increased  by  the  convolutions  of  the 
cortex.  Idiots'  brains  are  usually,  though  not  by  any  means 
invariably,  much  below  the  average  weight. 

A  difference  in  intellectual  vigor  may  be  present  in  persons 
whose  brain  shave  the  same  weight  and  even  the  same  amount  of 
gray  matter.  A  difference  in  the  quality  of  the  gray  substance 
may  in  such  cases  account  for  the  varying  results.  It  is  a  matter 
of  common  observation  that  mental  exercise  increases  mental 
vigor  and  capacity,  just  as  muscular  exercise  develops  muscular 
strength.  It  is  difficult  to  reach  a  conclusion  as  to  whether  there 
is  an  increase  in  the  amount  of  gray  substance  or  whether  that 
already  present  is  endowed  with  additional  power. 
18 


274  THE    NERVOUS    SYSTEM 

The  Cerebellum. 

Anatomy. — The  cerebellum,  or  little  brain  (see  Fig  79),  is 
situated  beneath  the  occipital  lobes  of  the  cerebrum,  weighs 
some  5 J  ounces  in  the  male  to  4^  ounces  in  the  female,  and  con- 
sists of  a  central  and  two  lateral  lobes.  It  is  composed  of  white 
and  gray  matter,  the  latter  being,  with  the  exception  of  the  corpora 
dentata  in  the  lateral  lobes,  situated  externally.  The  convolu- 
tions on  its  surface  are  much  finer  than  are  those  on  the  cerebral 
surface.  It  is  separated  from  the  parts  above  by  the  tentorium 
cerebelli,  a  process  of  the  dura  mater. 

Fibers. — The  fibers  passing  away  from  the  cerebellum  are  col- 
lected into  three  bundles  on  each  side,  known  as  the  superior, 
middle  and  inferior  peduncles.  The  superior  peduncle  has  a 
direction  forward  and  upward  to  reach  the  crus  and  optic  thala- 
mus;  fibers  in  it  connect  the  cerebellum  with  the  cerebrum. 
Certain  of  these  decussate  underneath  the  corpora  quadrigemina 
with  corresponding  fibers  from  the  opposite  side,  so  that  each 
side  of  the  cerebellum  is  connected  with  both  sides  of  the  cere- 
brum. Attention  has  been  called  to  fibers  passing  down  from 
the  cerebrum  through  the  pons  to  the  cerebellum.  Fibers  in  the 
middle  peduncle  connect  the  two  lateral  halves  of  the  cerebellum 
through  the  pons.  Fibers  in  the  inferior  peduncle  are  continuous 
below  with  fibers  in  the  posterior  columns  of  the  cord  through 
the  restiform  bodies  of  the  medulla. 

Function. — The  only  characteristic  phenomenon  invariably 
following  removal  of  the  cerebellum  is  an  inability  to  coordinate 
the  voluntary  muscular  movements.  The  foot,  for  example,  can  be 
raised,  and  the  voluntary  muscular  act  concerned  in  raising  it 
may  be  as  vigorous  as  ever,  but  the  animal  cannot  so  govern  his 
movements  as  to  know  where  he  put  it  down.  Even  the  coordin- 
ation necessary  in  standing  is  lost,  and  the  maintenance  of  the 
equilibrium  is  very  difficult,  if  not  impossible.  The  so-called 
muscular  sense  is  abolished,  and,  while  the  power  to  contract 


THE    CEREBELLUM  275 

the  muscles  remains,  the  animal  cannot  contract  them  in  a  regu- 
lar or  coordinate  manner.  When  it  is  remembered  that  well- 
nigh  every  voluntary  act  requires  concerted  or  consecutive  mus- 
cular movements  some  idea  is  gotten  of  the  helpless  condition 
sequent  upon  such  a  lesion.  If  it  be  granted  that  there  is  a  cen- 
ter presiding  over  the  coordination  of  the  voluntary  muscles, 
that  center  is  in  the  cerebellum,  and  an  animal  deprived  of  this 
organ  is  as  powerless,  so  far  as  this  function  is  concerned,  as  a 
person  is  to  see  when  the  optic  centers  are  destroyed.  Its  action 
is  crossed. 

It  has  been  noted  already  that  lesions  of  the  posterior  white 
columns  of  the  cord  are  followed  by  disturbances  of  coordination, 
and  that  the  cerebellum  is  connected  with  these  columns  through 
the  inferior  peduncles  and  restiform  bodies.  Fibers  in  these 
columns  serve  only  as  anatomical  connections  by  which  the  co- 
ordinating center  communicates  with  the  muscles  whose  move- 
ments it  is  to  regulate,  and  of  necessity  any  lesion  of  these  fibers 
destroying  that  connection  is  followed  by  the  loss  of  control  of  the 
center  over  the  muscles.  However,  in  degeneration  of  the  pos- 
terior columns  (locomotor  ataxia)  an  effort  at  coordination  can 
be  made,  so  that  progression  is  possible  by  the  aid  of  fixed  atten- 
tion. It  is  possible  also  that  the  coordinating  messages  are 
carried  in  such  cases  by  the  motor  fibers,  though  in  an  unsatis- 
factory manner. 

It  has  been  supposed  that  the  cerebellum  is  in  some  way  con- 
nected with  the  generative  function,  and  this  much  is  probably 
true,  though  the  evidence  submitted  is  not  sufficient  to  warrant 
the  assumption  that  the  cerebellum  is  the  seat  of  the  sexual 
instinct. 

THE  CRANIAL  NERVES. 

The  cranial  nerves,  twelve  in  number  on  each  side,  take  their 
origin  from  some  part  of  the  encephalon,  pierce  the  dura  mater 
and  leave  the  skull  by  various  openings.  They  have  been  num- 


276  THE   NERVOUS    SYSTEM 

bered  from  before  backward  in  the  order  in  which  they  pass 
through  the  dura  mater.  Their  names,  indicating  something  of 
their  function,  and  corresponding  to  their  numbers,  are  as  follows: 

I.  Olfactory. 
II.  Optic. 

III.  Motor  Oculi  Communis. 

IV.  Patheticus  (Trochlearis) . 
V.  Trifacial  (Trigeminus) . 

VI.  Abducens. 
VII.  Facial. 
VIII.  Auditory. 
IX.  Glosso-pharyngeal. 
X.  Pneumogastric  (Vagus). 
XI.  Spinal  Accessory. 
XII.  Hypoglossal. 

The  point  at  which  one  of  these  nerves  can  be  seen  to  issue 
from  the  brain  tissue  is  the  apparent  origin,  while  the  gray  nu- 
cleus, or  nuclei,  to  which  the  fibers  can  be  traced  in  the  brain 
substance  is  the  deep  origin. 

First  Nerve  (Olfactory). 

Origin. — This  is  a  nerve  of  special  sense.  Its  apparent  or- 
igin is  by  three  roots.  The  internal  root  issues  from  the  gyrus 
f ornicatus ;  the  middle  from  the  under  surface  of  the  frontal  lobe 
anterior  to  the  anterior  perforated  space;  the  external  from  the 
temporo-sphenoidal  lobe.  These  three  roots  unite  to  pass  for- 
ward underneath  the  frontal  lobe  near  the  longitudinal  fissure 
as  the  olfactory  tract.  The  deep  origin  is  unsettled. 

Course  and  Distribution. — Reaching  the  upper  surface  of  the 
cribriform  plate  of  the  ethmoid,  the  olfactory  tract  expands  into 
the  olfactory  bulb,  from  the  under  surface  of  which  are  given 
off  the  special  nerve  fibers  of  the  sense  of  smell.  They  are  about 


THE    CRANIAL   NERVES  277 

twenty  in  number  and  pass  through  the  foramina  in  the  cribri- 
form plate  to  be  distributed  to  the  mucous  membrane  (Schnei- 
derian)  of  the  nose  in  three  sets — an  inner  to  the  upper  third  of 
the  septum,  a  middle  to  the  roof  of  the  nares,  and  an  outer  to  the 
superior  and  middle  turbinated  bones  and  the  ethmoid  in  front 
of  them.  The  fibers  are  non-medullated. 

Function. — The  olfactory  nerves  are  insensible  and  inexcit- 
able.  They  are  concerned  with  the  sense  of  smell  alone,  and 
their  integrity  is  necessary  to  the  preservation  of  that  sense. 
They  convey  to  the  brain  impressions  which  are  recognized  as 
odors  only.  Removal  of  the  olfactory  bulb  in  a  dog  is  evidently 
followed  by  a  loss  of  the  sense  so  characteristic  of  the  animal. 
Furthermore,  the  olfactory  bulbs  in  lower  animals  are  shown  to 
be  developed  in  proportion  to  the  acuteness  of  the  sense  of  smell. 

Second  Nerve  (Optic). 

Origin. — This  is  the  nerve  of  sight.  Its  apparent  origin  is 
from  the  anterior  part  of  the  optic  commissure.  The  optic  com- 
missure occupies  the  optic  groove  on  the  superior  surface  of  the 
sphenoid.  It  represents  the  union  of  the  two  optic  tracts  each 
of  which,  traced  backward,  is  found  to  divide  into  two  bands; 
the  external  takes  its  origin  from  the  external  geniculate  body, 
from  the  pulvinar  of  the  optic  thalamus  and  from  the  superior 
corpus  quadrigeminum;  the  internal  comes  from  the  internal 
geniculate  body.  These  two,  uniting,  cross  the  crusta  obliquely 
to  reach  the  optic  commissure,  or  chiasm.  In  the  commissure 
the  fibers  from  the  inner  margin  of  each  optic  tract  pass  to  the 
other  side  of  the  brain,  and  may  be  called  commissural  fibers 
between  the  internal  geniculate  bodies.  Some  fibers  anteriorly 
connect  the  two  optic  nerves  with  each  other  and  are  not  prop- 
erly part  of  the  chiasm,  but  connect  the  two  retinae.  The  outer 
fibers  of  each  tract  pass  to  the  nerve  of  the  same  side,  while 
the  central  fibers  decussate  in  the  commissure  with  similar  fibers 


278  THE   NERVOUS    SYSTEM 

from  the  other  tract  and  pass  thus  to  the  optic  nerve  of  the  op- 
posite side.  The  deep  origin  is  indicated  above. 

Course  and  Distribution. — Each  optic  nerve  leaves  the 
front  of  the  optic  chiasm  to  pass  out  of  the  cranium  and  enter 
the  orbital  cavity  by  the  optic  foramen.  Having  pierced  the 
sclerotic  and  choroid  coats  of  the  ball  it  expands  into  the  retina. 

Function. — The  optic  nerves  have  no  properties  other  than 
the  conveying  to  the  brain  of  the  special  impressions  of  sight. 
Stimulation  produces  neither  pain  nor  motion. 

Third  Nerve  (Motor  Oculi  Communis) . 

Origin. — The  third  is  a  motor  nerve.  Its  apparent  origin  is 
from  the  inner  surface  of  the  crus  just  in  front  of  the  pons  Va- 
rolii.  Its  deep  origin  is  in  a  nucleus  just  lateral  to  the  median 
line  beneath  the  aqueduct  of  Sylvius.  Here  decussation  with 
fibers  from  the  opposite  side  occurs.  The  fibers  pass  forward 
from  this  place  through  the  locus  niger  and  tegmentum  to  the 
point  of  apparent  origin. 

Course  and  Distribution. — Having  traversed  the  outer  aspect 
of  the  cavernous  sinus,  the  third  nerve  divides  into  two  branches 
which  leave  the  cranial  cavity  by  the  sphenoidal  fissure  between 
the  two  heads  of  the  external  muscle  of  the  eye.  The  superior 
division  is  distributed  to  the  superior  rectus  and  levator  palpe- 
brae  superioris ;  the  inferior  separates  into  three  branches,  one 
of  which  is  distributed  to  the  inferior  rectus,  another  to  the 
internal  rectus,  and  a  third  to  the  inferior  oblique.  From 
this  last  a  branch  is  given  off  to  the  lenticular  ganglion  to  form 
its  inferior  root. 

Functions. — This  nerve  has  no  function  other  than  to  supply 
motion  to  the  parts  to  which  it  is  distributed.  It  is  insensible 
at  its  root,  but  receives  filaments  from  the  fifth  in  the  cavernous 
sinus,  beyond  which  point  stimulation  produces  pain  as  well  as 
muscular  contractions.  The  phenomena  sequent  upon  section  of 
the  nerve  are  suggested  in  its  distribution,  (i)  There  is  ptosis, 


THE   CRANIAL  NERVES  279 

or  dropping  of  the  upper  lid ;  for  the  lid  is  kept  open  by  the  leva- 
tor  palpebrae  superioris.  (2)  There  is  external  strabismus, 
because  the  external  rectus  is  not  supplied  by  this  nerve  and  is 
unopposed  by  the  internal  rectus,  the  action  of  which  is  paralyzed. 
Diplopia  is  the  consequence.  (3)  There  is  inability  to  turn  the 
ball  except  in  an  outward  direction  because  the  muscles  producing 
movements  on  the  vertical  and  horizontal  axes  are  deprived  of  in- 
nervation.  (4)  There  is  inability  to  rotate  the  eye  in  certain  direc- 
tions on  the  antero-posterior  axis.  The  antagonist  of  the  inferior 
oblique  is  the  superior  oblique,  the  tendency  of  which  latter  is 
to  rotate  the  globe  so  as  to  make  the  pupil  look  downward  and 
outward.  When  the  inferior  oblique  is  paralyzed  the  superior 
oblique  is  unopposed,  it  is  impossible  to  rotate  the  ball  as  is 
usual  in  sidewise  movements  of  the  head,  and  double  vision  is  the 
result.  (5)  There  is  slight  protrusion  of  the  whole  ball  from  re- 
laxation of  the  muscles.  (6)  The  pupil  is  dilated  and  move- 
ments of  the  iris  are  interfered  with.  Stimulation  of  the  third 
nerve  contracts  the  pupil,  but  when  it  is  cut  the  pupil  does  not 
respond  to  light.  The  ciliary  nerves  controlling  the  movements 
of  the  iris  come  from  the  ophthalmic  ganglion  of  the  sympa- 
thetic; to  this  ganglion  goes  a  branch  from  the  third  nerve.  It  is 
known  that  the  action  of  the  sympathetic  cannot  be  divorced 
from  that  of  the  cerebro-spinal  system;  and  whether  this  influ- 
ence of  the  third  nerve  is  exerted  directly  upon  the  iris  or  in- 
directly through  the  ophthalmic  ganglion  is  a  matter  of  some 
obscurity.  The  fact  that  the  action  of  the  iris  is  not  instanta- 
neous strongly  suggests  control  by  the  sympathetic. 

The  decussation  under  the  aqueduct  of  Sylvius  is  evidenced  by 
the  reflex  contraction  of  the  pupil  on  the  opposite  side  when  the 
central  end  of  a  divided  optic  nerve  is  stimulated.  The  impulse 
is  reflected  through  the  third  nerve.  It  is  not  to  be  understood, 
however,  that  the  motor  oculi  is  the  only  nerve  capable  of  influ- 
encing movements  of  the  iris.  Section  of  the  sympathetic  in 
the  neck  contracts  the  pupil,  even  after  section  of  the  third. 


280  THE   NERVOUS    SYSTEM 

Fourth  Nerve  (Patheticus). 

Origin. — This  is  a  purely  motor  nerve.  Its  apparent  origin 
is  behind  the  corpora  quadrigemina  from  the  valve  of  Vieussens. 
The  two  nerves  decussate  above  this  valve.  Its  deep  origin  is 
just  below  that  of  the  third  nerve  beneath  the  aqueduct  of  Sylvius. 

Course  and  Distribution. — Emerging  from  the  valve  of  Vie- 
ussens the  nerve  winds  around  the  superior  peduncle  of  the  cere- 
bellum and  the  crusta  immediately  above  the  pons,  and  passes 
forward  near  the  outer  wall  of  the  cavernous  sinus  to  find  exit 
from  the  cranial  cavity  by  the  sphenoidal  fissure.  Having  en- 
tered the  orbit,  it  runs  forward  to  be  distributed  to  the  orbital 
surface  of  the  superior  oblique.  In  the  cavernous  sinus  it 
receives  fibers  from  the  ophthalmic  division  of  the  fifth  and  from 
the  sympathetic,  and  occasionally  gives  off  a  branch  to  the  lach- 
rymal nerve. 

Function. — It  supplies  motor  power  to  the  superior  oblique 
muscle  alone.  Remembering  the  origin  and  attachment  of  this 
muscle  it  is  not  difficult  to  fortell  the  consequence  of  lesions  of  the 
nerve.  The  action  of  the  superior  oblique  is  to  rotate  the  ball 
upon  an  oblique  horizontal  axis  so  that  the  pupil  will  look  down- 
ward and  outward.  This  movement  cannot  be  accomplished 
when  the  nerve  is  cut,  and  the  inferior  oblique  asserts  itself  un- 
duly to  bring  about  an  opposite  effect.  The  ball  cannot  accom- 
modate itself  to  movements  of  the  head  toward  the  shoulder,  and 
double  vision  supervenes — unless  the  object  be  brought  in  the 
involuntary  line  of  vision  of  the  affected  eye. 

Fifth  Nerve  (Trifacial,  Trigeminus) . 

The  fifth  is  analogous  to  the  spinal  nerves  (i)  in  rising  by  two 
roots,  (2)  in  having  a  ganglion  on  its  posterior  root,  and  (3)  in 
having  a  mixed  function.  The  anterior  root  is  small  and  motor ; 
the  posterior  large  and  sensory. 

Origin. — Its  apparent  origin  is  from  the  side  of  the  pons  above 


THE    CRANIAL   NERVES  281 

the  median  line.  The  deep  origin  of  the  large,  sensory  root  is  in 
the  pons  immediately  below  the  floor  of  the  fourth  ventricle  and 
just  internal  to  its  marginal  boundary.  The  small,  motor  root 
rises  from  a  point  just  internal  to  the  large  root. 

Course  and  Distribution. — The  two  roots,  taking  their  origin 
as  above  described,  pass  through  the  dura  above  the  internal 
auditory  meatus  and  run  along  the  superior  border  of  the  petrous 
portion  of  the  temporal  bone  to  a  point  near  its  apex,  where  a 
large  ganglion,  the  semilunar  or  Gasserian,  is  developed  on  the 
posterior  root  and  occupies  a  depression  on  the  bone  for  its  re- 
ception. The  motor  root  passes  beneath  the  ganglion  without 
being  connected  with  it. 

The  posterior  root  will  be  first  followed  to  its  distribution. 

From  the  anterior  surface  of  the  Gasserian  ganglion  are  given 
off  three  branches — (i)  ophthalmic,  (2)  superior  maxillary, 
(3)  inferior  maxillary.  After  the  inferior  maxillary  has  left 
the  cranial  cavity  it  receives  fibers  from  the  small  or  motor  root, 
but  the  other  branches  are  composed  entirely  of  fibers  from  the 
sensory  root. 

i.  The  Ophthalmic  Branch  passes  forward  along  the  outer 
wall  of  the  cavernous  sinus,  divides  into  three  branches — (a) 
lachrymal,  (b)  frontal,  (c)  nasal — and  enters  the  orbit  by 
the  sphenoidal  fissure.  It  communicates  with  the  cavernous 
sympathetic,  third  and  sixth  nerves,  (a)  The  lachrymal 
branch,  running  along  the  outer  wall  of  the  orbit,  reaches  the 
lachrymal  gland,  gives  off  filaments  to  it  and  to  the  conjunctiva/ 
and  pierces  the  tarsal  ligament  to  be  finally  distributed  to  the 
integument  of  the  upper  lid.  (b)  The  frontal  branch  runs 
along  the  upper  wall  of  the  orbit  and  separates  into  the  supra- 
trochlear  and  supra-orbital  branches.  The  former  of  these 
leaves  the  orbit  in  front  and  turns  up  over  the  bone  to  supply  the 
integument  of  the  lower  forehead;  the  latter  traverses  the  supra- 
orbital  canal,  escapes  by  the  foramen  of  the  same  name,  and 
supplies  the  skin  as  far  back  as  the  occiput  as  well  as  the  peri- 


282  THE   NERVOUS    SYSTEM 

cranium  in  the  frontal  and  parietal  regions,  (c)  The  nasal 
branch,  crossing  to  the  inner  wall  of  the  orbit,  enters  the  anterior 
ethmoidal  foramen,  passes  thus  into  the  cranium  again,  runs  in  a 
groove  on  the  cribriform  plate  of  the  ethmoid  and  finds  exit  into 
the  nose  through  a  slit  by  the  side  of  the  crista  galli.  Here  it 
gives  off  branches  which  supply  common  sensation  to  the  mucous 
membrane  of  the  fore  part  of  the  nose,  and  then  running  in  a 
groove  on  the  posterior  surface  of  the  nasal  bone,  it  leaves  the 
cavity  at  the  lower  border  of  that  bone  to  supply  the  integument 
of  the  ala  and  tip  of  the  nose.  From  the  nasal  nerve  pass  fibers 
to  the  ophthalmic  ganglion  and  to  the  ciliary  muscle,  iris  and 
cornea. 

2.  The  Superior  Maxillary  Branch  passes  away  from  the 
Gasserian  ganglion  and  leaves  the  cranium  by  the  foramen  ro- 
tundum.  Crossing  the  spheno-maxillary  fossa  it  enters  the  orbit 
through  the  spheno-maxillary  fissure  and  traverses  the  infra- 
orbital  canal  to  emerge  upon  the  face  at  the  infra-orbital  foramen. 
In  the  cranium  it  gives  off  a  meningeal  branch  to  supply  the 
neighboring  dura  mater.  In  the  spheno-maxillary  fossa  it  sup- 
plies branches  (a)  to  the  integument  over  the  temporal  and  post- 
frontal  regions  and  over  the  cheeks;  (b)  to  the  spheno-palatine 
ganglion ;  (c)  the  posterior  superior  dental  branches  (generally 
two),  which  enter  the  posterior  dental  canals  in  the  zygomatic 
fossa,  and,  passing  forward  in  the  substance  of  the  superior  max- 
illa, give  off  twigs  to  the  fangs  of  the  molar  teeth,  supplying  them 
with  sensation.  In  the  infra-orbital  canal  the  superior  maxillary 
nerve  gives  off  (a)  the  middle  superior  dental,  which  runs  down- 
ward and  forward  in  the  outer  wall  of  the  antrum  to  reach  the 
roots  of  the  bicuspid  teeth;  (b)  the  anterior  superior  dental, 
which  likewise  runs  in  the  outer  wall  of  the  antrum  to  supply  the 
incisor  and  canine  teeth.  After  its  exit  from  the  infra-orbital 
canal  the  nerve  divides  into  palpebral,  nasal  and  labial 
branches,  which  supply  sensation  to  the  regions  indicated  by 
their  names. 


THE    CRANIAL   NERVES  283 

3.  The  Inferior  Maxillary  Branch  after  its  exit  from  the 
cranium  is  a  mixed  nerve,  supplying  motion  to  the  muscles  of 
mastication  as  well  as  common  sensation  to  the  parts  presently 
to  be  noted,  and  special  sense  to  a  part  of  the  tongue.  Its  large 
or  sensory  root  comes  from  the  Gasserian  ganglion  to  be  joined 
just  beneath  the  base  of  the  skull  by  the  small  motor  root  which 
has  passed  under  the  ganglion.  Almost  immediately  this  com- 
mon trunk  divides  into  (a)  anterior  and  (b)  posterior  branches, 
but  first  gives  off  a  recurrent  meningeal  branch  and  a  branch  to 
the  internal  pterygoid  muscle. 

(a)  The  anterior  of  the  two  divisions  of  the  inferior  maxillary 
nerve  receives  nearly  the  whole  of  the  motor  root  and  divides 
into  branches  which  supply  the  muscles  of  mastication,  except- 
ing the  internal  pterygoid  and  the  buccinator. 

(b)  The  posterior  division,  chiefly  sensory,  divides  into  the 
auriculo-temporal,   lingual  and  inferior  dental  branches. 
The   auriculo-temporal   branch   runs   backward   to   a  point 
internal  to  the  neck  of  the  condyle  of  the  inferior  maxilla,  then 
passing  upward  under  the  parotid  gland  divides  into  branches, 
which  are  distributed  to  the  external  auditory  meatus,  parotid 
gland,  integument  of  the  temporal  region  and  of  the  ear  and 
surrounding  parts.     It  communicates  with  the  otic  ganglion. 
The  lingual  branch  is  joined  by  the  chorda  tympani,  passes  to 
the  inner  side  of  the  ramus  of  the  jaw,  crosses  Wharton'sduct, 
and  is  distributed  to  the  papillae  and  mucous  membrane  of  the 
tongue  and  mouth.     It  communicates  with  the  facial  through 
the  chorda  tympani,  with  the  hypoglossal,  and  with  the  sub- 
maxillary  ganglion.     The  inferior  dental  branch  passes  be- 
tween the  internal  lateral  ligament  and  ramus  of  the  jaw  to  enter 
the  inferior  dental  foramen.     Thence  it  traverses  the   dental 
canal  in  the  inferior  maxilla  to  issue  at  the  mental  foramen. 
Here  it  divides  into  incisor  and  mental  branches;  the  former  con- 
tinues in  the  bone  to  supply  the  incisor  and  canine  teeth;  the 
latter  supplies  the  skin  of  the  chin  and  lower  lip.     In  its  course 


284  THE    NERVOUS    SYSTEM 

the  inferior  dental  gives  off  the  mylo-hyoid  (before  entering  the 
canal)  to  the  mylo-hyoid  and  anterior  belly  of  the  digastric,  and 
dental  branches  to  supply  the  molar  and  bicuspid  teeth. 

Four  small  ganglia,  usually  classed  as  part  of  the  sympathetic 
system,  are  connected  with  the  three  divisions  of  the  trifacial 
nerve.  The  ophthalmic,  or  lenticular,  ganglion  is  connected 
with  the  first  division;  the  spheno-palatine  or  MeckePs  with 
the  second;  the  otic  and  submaxillary  with  the  third.  All 
these  receive  sensory  fibers  from  the  trifacial  and  motor  fibers 
from  various  sources. 

Functions. — It  is  seen  from  the  foregoing  description  that 
the  trifacial  is  the  great  sensory  nerve  of  the  head  and  face,  and 
the  motor  nerve  of  the  muscles  of  mastication.  The  small,  or 
motor,  division  has  properly  been  called  the  "nerve  of  mastica- 
tion." It  is  insensible  upon  stimulation  before  it  is  joined  by  the 
third  division  of  the  sensory  root.  Its  section  causes  paralysis 
of  the  muscles  of  mastication  on  that  side.  It  cannot  be  doubted 
that  the  large  root  is  exclusively  sensory  at  its  origin,  and  the 
acuteness  of  that  sensibility,  as,  e.  g.,  in  the  teeth,  is  a  matter  of 
common  observation.  Immediate  loss  of  sensibility  in  the  area 
of  its  distribution  follows  section,  and  even  the  cornea,  which  is 
normally  exquisitely  sensitive,  can  be  touched  without  exciting 
pain.  Both  roots  are  usually  cut  at  the  same  time,  and  besides 
a  loss  of  motion  and  general  sensibility,  section  of  this  nerve 
produces  a  decided  effect  upon  the  eye,  the  sense  of  taste,  deglu- 
tition and  the  nutrition  of  the  parts  to  which  the  nerve  is  distrib- 
uted. The  flow  of  tears  is  increased,  the  pupil  becomes  tem- 
porarily contracted  and  the  ball  protrudes.  In  a  few  hours 
congestion  is  marked,  and  in  a  day  or  two  the  cornea  sloughs  and 
the  eye  is  destroyed.  Section  ot  the  fifth  before  its  lingual 
branch  is  joined  by  the  chorda  tympani  from  the  facial  causes 
a  loss  of  general  sensation,  but  not  of  taste,  in  the  anterior  part  of 
the  tongue;  section  of  the  lingual  branch  after  it  has  received  the 
chorda  is  followed  by  loss  of  general  sensation  and  of  taste.  This 


THE   CRANIAL  NERVES  285 

shows  that  the  special  sensibility  distributed  to  the  tongue  by 
the  lingual  branch  of  the  fifth  is  furnished  by  the  chorda  tympani. 
The  fifth  nerve  sends  filaments  to  give  sensibility  to  the  velum 
palati.  The  reflex  act  of  deglutition  is  due  to  impressions  carried 
from  the  velum  and  neighboring  parts  to  the  centers;  when  the 
fitth  nerve  is  cut  no  such  impressions  are  conveyed  and  the  reflex 
act  cannot  be  excited. 

Regarding  nutrition  it  is  noticed  that,  besides  the  sloughing  of 
the  cornea,  there  is  also,  about  the  same  time,  the  appearance  of 
ulcers  in  the  mouth  and  on  the  tongue,  and  animals  thus  experi- 
mented upon  soon  die.  These  lesions  are  much  less  marked 
when  the  section  is  behind  the  semilunar  ganglion.  Explanations 
of  this  difference  are  not  altogether  satisfactory,  but  it  is  rational 
to  suppose  that  section  of  sympathetic  fibers  when  the  nerve 
is  cut  in  front  of  Gasser's  ganglion  is  responsible  for  the  dis- 
turbances of  nutrition;  for  this  is  the  system  of  nutrition,  and 
changes  following  its  section  in  other  parts  of  the  body  are  not 
unlike  those  under  discussion.  Why,  however,  the  changes 
should  be  inflammatory  in  character  is  not  explained  by  this  hy- 
pothesis, unless  it  be  an  explanation  to  say  that  the  inflammation 
is  set  up  by  the  impairment  of  nutrition  in  these  structures — the 
impairment  resulting  in  part  from  the  impoverished  condition  of 
the  blood  as  a  consequence  of  the  inability  of  the  animal  to  chew. 

Sixth  Nerve  (Abducens). 

Origin. — This  is  a  motor  nerve  entirely.  Its  apparent  origin 
is  from  the  lower  border  of  the  pons  in  the  groove  separating  it 
from  the  anterior  pyramid  of  the  medulla.  Its  deep  origin  is 
close  to  the  median  line  beneath  the  floor  of  the  fourth  ventricle 
a  little  below  the  motor  root  of  the  fifth. 

Course  and  Distribution. — The  nerve  enters  the  cavernous 
sinus,  runs  forward  to  enter  the  orbit  by  the  sphenoidal  fissure, 
passes  between  the  two  heads  of  the  external  rectus,  and  is  dis- 
tributed to  the  ocular  surface  of  that  muscle.  In  the  cavernous 


286  THE   NERVOUS    SYSTEM 

sinus  it  receives  fibers  from  the  first  division  of  the  fifth  and  from 
the  sympathetic. 

Function. — The  function  is  indicated  in  its  distribution.  It 
is  insensible  at  its  origin.  Stimulation  produces  contraction  of 
the  external  rectus;  section  causes  paralysis  of  that  muscle  and 
consequent  internal  strabismus  and  diplopia. 

Seventh  Nerve  (Facial). 

Origin. — The  apparent  origin  of  the  seventh  is  from  the  up- 
per end  of  the  medulla  in  the  groove  between  the  olivary  and 
restiform  bodies.  Its  deep  origin  is  in  the  pons  beneath  the 
floor  of  the  fourth  ventricle  a  little  external  to  the  nucleus  of  the 
sixth. 

Course  and  Distribution. — The  seventh  nerve  passes  out- 
ward and  forward  with  the  auditory  nerve  (on  its  inner  side)  to 
enter  the  internal  auditory  meatus.  From  their  relative  firmness 
and  texture  and  their  close  relation  here,  the  seventh  and  eighth 
nerves  have  been  called  respectively  the  portio  dura  and  the 
portio  mollis.  Running  between  them  is  a  fasciculus  from  the 
medulla  known  as  the  intermediary  nerve  of  Wrisberg,  or  the 
portio  inter  duram  et  mollem ;  most  of  its  fibers  join  the  facial 
in  the  internal  auditory  meatus.  The  facial  nerve  enters  the 
Fallopian  aqueduct  at  the  bottom  of  the  meatus  and  follows 
it  to  issue  at  the  stylo-mastoid  foramen,  run  forward  in  the  sub- 
stance of  the  parotid  gland  and  divide  behind  the  ramus  of  the 
jaw  into  temporo-facial  and  cervico -facial  branches. 

Its  branches  of  communication  are  numerous,  (i)  In  the  in- 
ternal auditory  meatus  it  communicates  with  the  auditory  nerve ; 
(2)  in  the  aqiieductus  Fallopii  with  the  otic  and  spheno-palatine 
ganglia,  with  the  sympathetic  and  with  the  auricular  branch  of 
the  pneumogastric;  (3)  after  leaving  the  stylo-mastoid  foramen, 
with  the  fifth,  ninth,  tenth  and  sympathetic. 

Its  branches  of  distribution  are  also  quite  numerous,  (i)  In 
the  aqueductus  Fallopii  it  gives  off  (a)  the  tympanic  branch  to 


THE   CRANIAL   NERVES  287 

the  stapedius  muscle,  and  (b)  the  chorda  tympani,  which  passes 
through  the  cavity  of  the  tympanum  and  emerges  by  a  foramen  at 
the  inner  end  of  the  Glaserian  fissure  to  go  to  the  lingual  branch 
of  the  fifth.  (2)  At  its  exit  from  the  stylo-mastoid  foramen  it 
gives  off  (a)  a  posterior  auricular  branch  which,  receiving  a 
filament  from  the  auricular  branch  of  the  tenth,  is  distributed  to 
the  retrahens  aurem  and  the  occipital  portion  of  the  occipito- 
frontalis;  (b)  a  digastric  branch  to  the  posterior  belly  of  the  di- 
gastric muscle;  (c)  a  stylo-hyoid  branch  to  the  muscle  of  that 
name.  (3)  On  the  face  it  divides  into  (a)  a  temporo-facial 
branch,  which  is%  distributed  to  the  muscles  over  the  temple  and 
upper  face;  and  (b)  a  cervico-facial  branch,  which  is  distributed 
to  the  lower  face  and  upper  cervical  region. 

Functions. — This  is  the  motor  nerve  of  the  muscles  of  ex- 
pression, of  the  platysma,  buccinator,  digastric  (posterior  belly) , 
stylo-hyoid,  the  muscles  of  the  external  ear  and  the  stapedius. 
Communicating  freely  with  the  fifth,  it  also  contains  sensory 
fibers,  but  it  is  in  all  probability  insensible  at  its  root.  Its  sec- 
tion causes  paralysis  of  the  muscles  which  it  supplies,  but  no 
marked  changes  in  sensation.  The  branches  to  the  otic  and 
spheno-palatine  ganglia  in  the  aqueductus  Fallopii  constitute 
their  motor  roots;  the  branch  given  off  in  this  situation  to  the 
tenth  supplies  it  with  motor  filaments,  and  probably  also  here  pass 
sensory  fibers  from  the  tenth  to  the  seventh.  In  facial  paralysis 
when  the  lesion  is  in  the  aqueductus  Fallopii  or  behind  it,  there 
is  paralysis  also  of  the  muscles  of  the  palate  and  uvula,  the  uvula 
is  drawn  to  the  opposite  side  and  there  is  trouble  in  deglutition. 
The  fibers  to  the  azygos  uvulae  and  levator  palati  pass  from 
the  aqueductus  Fallopii  through  Meckel's  ganglion. 

The  effect  of  paralysis  of  the  facial  upon  the  superficial  muscles 
of  the  face  is  suggested  in  its  distribution.  The  brow  cannot  be 
corrugated;  the  eye  is  constantly  open  and  there  may  be  con- 
sequent inflammation  from  exposure;  the  nostril  cannot  be  di- 
lated, and  inspiration  and  possibly  olf action  are  interfered  with; 


288  THE   NERVOUS    SYSTEM 

the  cheek  is  flaccid;  the  lips  are  immobile  and  saliva  may  flow 
from  that  corner  of  the  mouth;  the  buccinator  is  paralyzed,  and 
there  is  often  great  difficulty  in  mastication  because  of  the  accu- 
mulation of  food  between  the  cheek  and  the  teeth;  the  unop- 
posed action  of  the  muscles  of  the  opposite  side  greatly  distort 
the  facial  features,  the  affected  side  being  quite  expressionless. 
Facial  monoplegia  is  common;  facial  diplegia  is  very  uncommon. 
The  Chorda  Tympani. — This  branch  of  the  seventh  is  con- 
cerned especially  in  gustation.  The  fibers  of  which  it  is  com- 
posed undoubtedly  come  from  nerve  of  Wrisberg.  Section  of 
the  seventh  involving  also  the  nerve  of  Wrisberg  causes  not  only 
facial  palsy  but  also  a  loss  of  the  sense  of  taste  in  the  anterior 
two-thirds  of  the  tongue.  The  sense  of  taste  will  receive  later 
notice. 

Eighth  Nerve  (Auditory) . 

Origin. — This  is  a  nerve  of  special  sense.  Its  apparent 
origin  is  by  two  roots — one  from  the  groove  between  the  olivary 
and  restiform  bodies  at  the  lower  border  of  the  pons,  the  other 
coming  around  the  upper  end  of  the  restiform  body  to  join  the 
first  in  the  groove.  The  deep  origin  of  the  two  roots  is  different. 
That  of  the  median  root  is  the  dorsal  auditory  nucleus  in  the 
floor  of  the  fourth  ventricle;  that  of  the  lateral  root  is  mainly 
from  the  ventral  auditory  nucleus  in  front  of  the  restiform  body 
between  the  two  roots. 

Course  and  Distribution. — Crossing  the  posterior  border  of 
the  middle  peduncle  of  the  cerebellum,  it  enters  the  internal  audi- 
tory meatus  in  company  with  the  facial  nerve  and  the  nerve  of 
Wrisberg.  At  the  bottom  of  the  meatus  it  receives  fibers  from 
the  seventh,  and  divides  into  branches  which  pass  to  the  cochlea, 
semicircular  canals  and  vestibule. 

Function. — This  nerve  receives  and  conveys  to  the  brain  im- 
pressions produced  by  sound  waves;  it  is  the  nerve  of  hearing 


THE    CRANIAL   NERVES  289 

and  is  in  all  probability  not  sensible  to  stimulation  in  any  other 
way. 

Ninth  Nerve  (Glosso-pharyngeal). 

Origin. — The  apparent  origin  of  this  nerve  is  from  the  upper 
part  of  the  medulla  in  the  groove  between  the  olivary  and  resti- 
form  bodies.  Its  deep  origin  is  in  the  lower  part  of  the  floor  of 
the  fourth  ventricle  above  the  nucleus  of  the  tenth. 

Course  and  Distribution. — Leaving  the  skull  by  the  jugular 
foramen,  it  passes  forward  between  the  internal  jugular  vein  and 
the  internal  carotid  artery,  descends  in  front  of  the  latter  to  the 
lower  border  of  the  stylo-pharyngeus  where  it  curves  inward, 
runs  beneath  the  hyoglossus,  and  is  distributed  to  the  fauces,  pos- 
terior third  of  the  tongue,  and  the  tonsil. 

It  communicates  with  the  seventh,  tenth   and   sympathetic. 

Its  branches  of  distribution  go  to  the  mucous  membrane  and 
muscles  of  the  pharynx,  the  stylo-pharyngeus,  the  tonsil  and 
soft  palate,  the  circumvallate  papillae  and  the  mucous  membrane 
at  the  base  and  side  of  the  tongue  and  on  the  anterior  surface  of 
the  epiglottis.  Some  of  its  branches  join  branches  from  the 
pharyngeal  and  external  laryngeal  branches  of  the  pneumogas- 
tric  to  form  the  pharyngeal  plexus. 

Functions. — It  is  the  nerve  of  sensation  to  the  pharynx  and 
fauces  and  a  nerve  of  taste  to  the  base  of  the  tongue.  Its  sensi- 
bility at  its'  root  is  dull,  but  stimulation  produces  no  motion. 
Although  this  nerve  is  distributed  to  the  mucous  membrane  over 
the  base  of  the  tongue,  palate  and  pharynx,  these  parts  receive 
the  greater  portion  of  their  general  sensibility  from  filaments  of 
the  fifth,  and  section  of  the  ninth  produces  no  marked  effect 
upon  the  reflex  phenomena  of  deglutition.  The  sense  of  taste  is 
distributed  to  the  anterior  two-thirds  of  the  tongue  by  the  chorda 
tympani,  and  it  has  nothing  to  do  with  general  sensation,  while 
the  glosso-pharyngeal,  endowing  the  posterior  third  with  gusta- 
tory power,  also  furnishes  to  it  a  degree  of  general  sensibility. 
19 


2QO  THE   NERVOUS   SYSTEM 

Tenth  Nerve  (Pneumogastric,  Vagus). 

Origin. — This  is  a  mixed  nerve.  Its  apparent  origin  is  from 
the  groove  between  the  olivary  and  restiform  bodies  below  the 
ninth.  Its  deep  origin  is  in  the  floor  of  the  fourth  ventricle 
just  below  that  of  the  glasso-pharyngeal. 

Course  and  Distribution. — As  it  leaves  the  cranium  by  the 
jugular  foramen  it  presents  a  ganglionic  enlargement,  the  jugu- 
lar ganglion,  or  ganglion  of  the  root,  just  below  which  it  is 
joined  by  the  accessory  portion  of  the  spinal  accessory.  Below 
the  junction  is  a  second  ganglion,  the  ganglion  of  the  trunk. 
The  accessory  part  of  the  eleventh  passes  through  this  ganglion, 
and  below  unites  with  the  vagus  trunk  to  pass  chiefly  into  its 
pharyngeal  and  superior  laryngeal  branches.  The  pneumogas- 
tric  passes  down  the  neck  behind  and  between  the  internal  jugu- 
lar vein  and  the  internal  and  common  carotid  arteries,  and  sends 
motor  and  sensory  fibers  to  the  organs  of  voice  and  respiration, 
and  motor  fibers  to  the  pharynx,  esophagus,  stomach  and  heart. 

The  branches  of  the  pneumogastric  are  numerous,  (i)  In 
the  jugular  fossa  it  gives  off  (a)  ameningeal  branch  to  the  dura 
mater  of  the  posterior  fossa  of  the  skull ;  (b)  an  auricular  branch 
which,  traversing  the  substance  of  the  temporal  bone,  emerges 
by  the  auricular  fissure  to  supply  the  integument  of  the  back 
part  of  the  pinna  and  external  auditory  meatus.  (2)  In  the 
neck  it  gives  off  (a)  a  pharyngeal  branch,  which  consists  mainly 
of  fibers  from  the  accessory  portion  of  the  eleventh  and  is  the 
chief  motor  nerve  of  the  pharynx  and  soft  palate;  (b)  a  superior 
laryngeal  branch,  which  also  consists  mainly  of  fibers  from  the 
accessory  part  of  the  eleventh  and  is  the  chief  sensory  nerve 
of  the  larynx;  it  also  animates  the  crico-thyroid  muscle;  (c)  a 
recurrent  laryngeal  branch,  which,  on  the  right  side,  winds 
round  the  subclavian  artery,  and,  on  the  left,  round  the  aorta  to 
return  to  the  muscles  of  the  larynx  whose  motor  nerve  it  is;  (d) 
cervical  cardiac  branches,  which  communicate  with  the  cardiac 


THE   CRANIAL   NERVES  2QI 

branches  ot  the  sympathetic  and  pass  to  the  deep  cardiac  plexus. 
(3)  In  the  thorax  it  gives  off  (a)  thoracic  cardiac  branches, 
which  pass  to  the  deep  cardiac  plexus ;  (b)  anterior  pulmonary 
branches,  which  go  to  the  roots  of  the  lungs  in  front;  (c)  pos- 
terior pulmonary  branches,  which  go  to  the  roots  of  the  lungs 
behind  and  send  some  filaments  to  the  pericardium;  filaments 
from  (b)  and  (c)  follow  the  air  passages  through  the  lungs;  (d) 
esophageal  branches,  which  unite  with  fibers  from  the  opposite 
nerve  to  form  the  esophageal  plexus.  (4)  In  the  abdomen  are 
the  gastric  branches ;  those  from  the  left  nerve  are  distributed 
to  the  anterior  surface  of  the  stomach,  and  those  from  the  right 
to  the  posterior;  the  right  vagus  is  also  distributed  to  the  liver, 
spleen,  kidneys  and  entire  small  intestine. 

Throughout  the  whole  course  of  the  pneumogastric  commun- 
ication with  other  nerves,  especially  the  sympathetic,  is  very  free. 

Functions. — The  root  of  the  tenth  in  the  medulla  is  purely 
sensory,  but  the  nerve  communicates  with  at  least  five  motor 
nerves,  and  is  distributed  to  mucous  membranes  and  to  voluntary 
and  involuntary  muscle  tissue.  The  auricular  branches  contain 
both  motor  and  sensory  fibers,  and  their  function  is  indicated  in 
their  distribution.  The  pharyngeal  branches  are  mixed,  re- 
ceiving motor  filaments  from  the  spinal  accessory.  Sensibility 
is  supplied  to  the  pharynx  not  by  this  nerve  alone,  but  by  the 
branches  of  the  fifth  and  probably  of  the  ninth;  indeed  it  seems 
that  the  pharyngeal  branches  of  the  tenth  have  little  to  do  with 
the  reflex  phenomena  of  deglutition.  The  superior  laryngeal 
branches,  mainly  sensory,  supply  also  motor  power  to  the  crico- 
thyroids.  Stimulation  of  the  filaments  of  these  branches  pre- 
vents the  entrance  of  foreign  bodies  into  the  larynx  by  reflex 
closure  of  the  glottis,  and  also  excites  movements  of  deglutition. 
Their  section  produces  hoarseness.  The  recurrent,  or  inferior 
laryngeal,  branches,  chiefly  motor,  supply  the  muscular  tissue 
of  the  upper  esophagus  and  trachea,  as  well  as  the  muscles  of  the 
larynx.  Section  of  them  causes  embarassed  phonation,  though 


292  THE    NERVOUS    SYSTEM 

the  fibers  thus  influencing  the  vocal  sounds  come  to  the  recur- 
rent laryngeal  from  the  spinal  accessory.  The  uses  of  the  car- 
diac branches  have  been  noticed  under  discussion  of  the  heart's 
action.  The  pulmonary  branches  are  both  motor  and  sensory 
and  go  to  the  lower  trachea,  the  bronchi  and  lung  substance. 
Section  of  the  tenth  destroys  the  sensibility  of  the  mucous  mem- 
brane of  the  trachea  and  bronchi  and  the  contractile  power  of 
the  muscular  fibers  of  the  tubes.  The  esophageal  branches  are 
mixed,  though  motor  fibers  predominate.  Food  will  not  pass 
readily  into  the  stomach -on  section  of  the  tenth  because  of  the 
absence  of  muscular  contractions  in  the  esophagus. 

Influence  of  the  Vagus  on  Respiration. — Section  of  both 
these  nerves  temporarily  increases  the  number  of  respirations, 
which  soon,  however,  become  exceedingly  slow  until  death 
ensues.  Inspiration  is  very  profound — indeed  so  profound  as  to 
produce  rupture  of  some  of  the  pulmonary  capillaries  with  con- 
sequent hemorrhage  and  coagulation  of  the  blood  and  consolida- 
tion of  the  lung  in  part  or  whole.  Section  of  only  one  of  the  vagi 
is  not  usually  followed  by  death.  Further  notice  of  the  relation 
of  the  pneumogastric  to  respiration  is  given  elsewhere. 

Influence  of  Vagus  on  the  Stomach,  Intestine  and  Liver.— 
Stimulation  of  the  pneumogastric  causes  contraction  of  the  stom- 
ach; but  since  the  contraction  is  not  immediate,  the  impulse  is 
probably  carried  to  it  by  fibers  of  the  sympathetic  running 
with  the  gastric  branches  of  the  tenth.  When  the  vagus  is  cut 
during  digestion  in  the  stomach  the  contractions  of  the  muscular 
wall  are  impaired  and  the  sensibility  of  the  organ  is  abolished. 
Secretion  is  interfered  with,  but  not  stopped. 

Section  of  the  vagus  seems  also  to  impair  intestinal  secretion 
and  movements,  but  it  is  not  improbable  that  this  is  because 
sympathetic  fibers  joining  the  vagus  high  in  the  neck  are  dis- 
tributed with  it  to  the  intestine. 

Simple  division  of  the  pneumogastrics  inhibits  the  formation 
of  glycogen  in  the  liver;  but  when  the  central  ends  of  the  cut 


THE    CRANIAL   NERVES  293 

nerves  are  stimulated  there  is  an  increased  production  of  sugar 
even  to  the  point  of  glycosuria.  The  irritation  is  probably  re- 
flected through  the  sympathetic;  indeed  it  is  not  supposed  that 
the  vagi  are  concerned  in  the  glycogenic  function  of  the  liver,  ex- 
cept reflexly;  its  section  only  prevents  the  conduction  cephalad  of 
the  impressions  which  usually  give  rise  to  a  secretion  of  glycogen. 
The  connection  of  the  vagus  with  the  kidneys,  spleen  and 
suprarenal  capsules  is  obscure. 

Eleventh  Nerve  (Spinal  Accessory). 

Origin. — This  nerve  consists  of  a  cranial  portion,  accessory 
to  the  tenth,  and  a  spinal  portion.  The  apparent  origin  of  the 
cranial  root  is  from  the  side  of  the  medulla  just  below  the  vagus. 
Its  deep  origin  is  in  the  medulla  to  the  posterior  and  outer  side 
of  the  nucleus  of  the  ninth.  The  apparent  origin  of  the  spinal 
portion  is  by  several  filaments  from  the  side  of  the  cord  as  low 
down  as  the  sixth  cervical  nerve.  Its  deep  origin  is  from  a 
column  of  cells  in  the  anterior  cornu  of  gray  matter  of  the  cord. 

Course  and  Distribution  (Accessory  Portion). — Passing  out  to 
the  jugular  foramen  it  is  joined  by  the  spinal  portion,  and  sends 
a  few  filaments  to  the  ganglion  of  the  root  of  the  tenth;  then 
leaving  the  spinal  portion  it  finds  exit  from  the  cranium  by  the 
jugular  foramen,  passes  over  the  ganglion  of  the  trunk  of  the 
tenth  (adherent  to  it),  and  is, continued  chiefly  in  the  pharyngeal 
and  superior  laryngeal  branches  of  that  nerve  (Gray),  but  in  the 
recurrent  laryngeal  as  well. 

Spinal  Portion. — Running  upward  between  the  two  roots  of 
the  spinal  nerves  the  spinal  portion  enters  the  cranial  cavity  by 
the  foramen  magnum,  passes  outward  to  the  jugular  foramen, 
where  it  joins  the  accessory  portion  to  separate  from  it  on  pass- 
ing through  that  foramen.  After  leaving  the  skull  it  takes  a 
course  backward,  pierces  the  sterno-mastoid,  crosses  the  occipital 
triangle  and  terminates  in  the  trapezius.  It  gives  branches  to 
the  sterno-mastoid  and  to  the  cervical  plexus. 


294  THE   NERVOUS    SYSTEM 

Functions. — Both  roots  of  this  nerve  are  purely  motor,  but 
communication  with  other  nerves  gives  it  a  degree  of  sensibility. 
The  fibers  from  the  medulla  (accessory)  go  exclusively  to  the 
muscles  of  the  larynx  and  pharynx,  which  those  from  the  cord 
(spinal)  go  exclusively  to  the  sterno-mastoid  and  trapezius; 
and  section  of  either  root  separately  is  followed  by  phenomena 
corresponding  to  these  facts.  When  both  roots  are  divided 
there  is  loss  of  voice,  disturbance  of  deglutition,  loss  of  cardiac 
inhibition  and  partial  paralysis  of  the  sterno-mastoid  and  tra- 
pezius. The  loss  of  voice  and  disturbance  in  deglutition  are 
explained  by  the  distribution  of  the  fibers  of  the  eleventh  with 
the  pharyngeal  and  laryngeal  branches  of  the  tenth.  The  loss 
of  the  power  of  the  vagus  to  inhibit  cardiac  action  is  because  the 
fibers  of  the  tenth  which  convey  the  inhibitory  impulses  are  re- 
ceived from  the  spinal  accessory.  The  sterno-mastoid  and 
trapezius  are  only  partially  paralyzed  because  they  receive  motor 
fibers  also  from  the  cervical  plexus. 

Twelfth  Nerve  (Hypoglossal). 

Origin. — This  nerve  supplies  motion  to  the  tongue.  Its 
apparent  origin  is  by  10-15  filaments  in  the  groove  between  the 
anterior  pyramid  of  the  medulla  and  the  olivary  body.  Its 
deep  origin  is  in  the  floor  of  the  fourth  ventricle  under  the  lower 
border  of  the  fasciculus  teres. 

Course  and  Distribution. — The  nerve  passes  through  the 
anterior  condyloid  foramen  in  two  bundles  which  unite  to  form  a 
common  trunk  below.  Running  downward  in  company  with 
the  internal  carotid  artery  and  internal  jugular  vein,  it  reaches  a 
point  opposite  the  angle  of  the  jaw,  then  runs  forward,  crosses 
the  external  carotid,  lies  on  the  hyoglossus  and  is  continued  for- 
ward in  the  genio-hyoglossus  to  the  tip  of  the  tongue. 

It  communicates  with  the  tenth,  sympathetic,  first  and  second 
cervical  and  the  lingual  branch  of  the  fifth. 

Its  branches  of  distribution  are  (i)  meningeal  to  the  dura 


THE   CRANIAL   NERVES  295 

mater  in  the  posterior  fossa  of  the  skull;  (2)  descendens  hypo- 
glossi,  which,  running  downward  across  the  sheath  of  the  great 
vessels,  meets  branches  of  the  second  and  third  cervical  nerves 
to  form  a  loop  from  which  are  supplied  the  sterno-hyoid,  the 
omo-hyoid  and  the  sterno-thyroid  muscles;  (3)  thyro-hyoid  to 
the  muscle  of  that  name;  (4)  muscular  to  the  muscular  sub- 
stances of  the  tongue  and  to  the  styloglossus,  hyoglossus,  genio- 
hyoid  and  genio-hyoglossus  muscles. 

Functions. — This  nerve  possesses  no  sensibility  at  its  root, 
but  receives  sensory  fibers  from  anastomoses  with  other  nerves. 
Its  stimulation,  therefore,  causes  movements  of  the  tongue  and 
some  pain.  Section  of  both  nerves  causes  difficult  deglutition, 
loss  of  power  over  the  tongue  and  consequent  disturbances  in 
mastication  and  articulation.  When  the  twelfth  is  affected  in 
hemiplegia  the  tongue,  on  protrusion,  deviates  to  the  affected 
side  because  it  is  pushed  out  by  the  genio-hyoglossus. 

It  will  be  seen  from  the  foregoing  that,  classified  according  to 
their  properties  at  their  roots,  the  I.,  II.  and  VIII.  are  nerves  of 
special  sense;  the  III.,  IV.,  VI.,  XI.  and  XII.  are  motor;  the  X. 
is  sensory;  and  the  V.,  VII.  and  IX.  are  mixed.  It  is  to  be  remem- 
bered, however,  that  most  of  these  (excepting  the  nerves  of 
special  sense)  are  mixed  in  their  distribution  by  reason  of  the 
reception  of  fibers  from  other  nerves.  The  term  " mixed"  in  the 
above  classification  is  used  as  meaning  the  association  of  special 
sensory  fibers  with  motor  or  common  sensory  fibers  as  well  as  the 
association  of  these  latter  with  each  other.  The  VII.  is  classed 
as  a  mixed  nerve  only  by  allowing  that  the  intermediary  nerve  of 
Wrisbery  is  to  be  considered  a  part  of  it.  Its  own  proper  root  is 
purely  motor. 

THE  SPINAL  NERVES. 

The  spinal  nerves,  thirty-one  on  each  side,  are  so  called  from 
the  fact  that  they  originate  in  the  spinal  cord  and  escape  from  the 
spinal  canal  by  the  intervertebral  foramina.  Eight  pairs  come 


296 


THE   NERVOUS    SYSTEM 


from  the  cervical  region  of  the  column,  twelve  from  the  dorsal, 
five  from  the  lumbar,  five  from  the  sacral,  and  one  from  the 
coccygeal.  They  are  numbered  according  to  their  foramina  of 
exit. 

Each  nerve  rises  by  two  roots — an  anterior  which  can  be 
traced  to  the  anterior  cornu  of  gray  matter  and  a  posterior 
which  goes  (apparently)  to  the  posterior  cornu — and  these  emerge 


B. 


FIG.  87. 

A.  bipolar  cell  from  spinal  ganglion  of  a  4^  weeks'  embryo  (after  His) .  n,  nucleus  ; 
the  arrows  indicate  the  direction  in  which  the  nerve  processes  grow,  one  to  the  spinal 
cord,  the  other  to  the  periphery.  B,  a  cell  from  the  spinal  ganglion  of  the  adult;  the 
two  processes  have  coalesced  to  form  a  T-shaped  junction.  (Kirkes.) 

respectively  from  the  antero-lateral  and  postero-lateral  fissures  of 
the  cord.  Before  leaving  the  spinal  canal  these  two  roots  join  to 
pass  through  the  corresponding  ihtervertebral  foramen  as  a  single 
tiunk  which,  however,  just  beyond  that  foramen  divides  into 
anterior  and  posterior  branches  to  be  distributed  to  the  anterior 
and  posterior  parts  of  the  body. 

The  posterior  root  (inside  the  spinal  canal)  is  sensory,  and 
has  a  ganglion  developed  upon  it.  The  fibers  of  the  posterior 


THE    SPINAL   NERVES  297 

root  are  outgrowths  of  cells  in  the  ganglion  of  that  root,  as  indi- 
cated in  Fig.  87.  This  accounts  for  the  arborization  of  the  differ- 
ent fibers  around  cells  in  the  cord  instead  of  an  actual  connection 
with  them.  These  facts  should  not  be  lost  sight  of  though  it  is 
customary  to  speak  of  an  efferent  fiber  as  passing  directly  to  a 
cord  cell  itself.  The  anterior  root  is  entirely  motor  except  for 
a  degree  of  "recurrent"  sensibility  which  is  due  to  the  presence 
in  it  of  posterior  root  fibers  which  have  passed  backward  from 
the  point  of  junction  of  the  two  probably  to  supply  the  mem- 
branes of  the  cord.  The  common  trunk  is,  of  course,  mixed,  as 
are  the  anterior  and  posterior  branches  passing  from  it. 

These  spinal  nerves  are  distributed  to  the  muscles  of  the  trunk 
and  extremities,  to  the  integument  of  almost  the  entire  body 
and  to  some  mucous  membranes;  and  from  what  has  been  said 
in  speaking  of  the  cord  about  the  connection  between  it  and 
these  nerves,  and  their  connection  through  it  with  the  higher 
centers,  it  is  evident  that  they  are  most  important  factors  which, 
acting  under  the  guidance  of  the  sensorium,  on  the  one  hand, 
tell  of  the  condition  of  the  organism — its  relations  and  environ- 
ments— and,  on  the  other,  control  the  voluntary  movements  of 
the  body. 

The  spinal  nerve  fibers  come  in  part  directly  from  the  brain 
and  in  part  from  the  gray  cells  of  the  cord. 

THE  SYMPATHETIC  SYSTEM. 

The  sympathetic  has  been  separated  from  the  cerebro-spinal 
system  only  for  the  sake  of  convenience.  The  former  sends 
filaments  to  the  latter  and  receives  both  motor  and  sensory  fibers 
in  return,  while  the  cooperation  of  the  two  systems,  regulating  in 
harmony  all  the  physiological  processes  going  on  in  the  body, 
is  too  evident  to  be  questioned. 

The  sympathetic  system  is  remarkable  for  the  number  of 
ganglia  connected  with  it.  These  may  be  divided  into  (a}  those 
along  the  vertebral  column,  as  the  thoracic,  (6)  those  in  close 


298  THE   NERVOUS    SYSTEM 

proximity  to  the  viscera  and  from  which  those  viscera  are  to  be 
directly  supplied,  as  the  semilunar,  and  (c)'  terminal  ganglia 
which  the  fibers  reach  just  before  final  distribution,  as  the  car- 
diac, intestinal,  etc.  The  sympathetic  is,  therefore,  frequently 
known  as  the  ganglionic  system. 

Arrangement. — There  is  on  each  side  of  the  spinal  column, 
extending  from  the  lenticular  ganglion  above  to  the  ganglion 
impar  below,  a  chain  of  ganglia  all  of  which  are  united  to  each 
other  and  to  the  ganglia  of  the  opposite  chain  by  commissural 
fibers.  From  these  ganglia  go  fibers  to  form  numerous  plexuses 
and  to  be  distributed  to  the  various  parts.  In  the  skull  there 
are  four  of  these  ganglia,  the  otic,  ophthalmic,  submaxillary 
and  spheno-palatine  or  Meckel's;  in  the  cervical  region  there  are 
three ;  in  the  dorsal  twelve ;  in  the  lumbar  four ;  in  the  sacral  four 
or  five;  and  in  front  of  the  coccyx  the  single  ganglion  impar. 

Connections  between  the  cranial  nerves  and  cranial  sympa- 
thetic ganglia  have  already  been  noted. 

The  cervical  ganglia  are  of  special  interest  as  furnishing  the 
chief  sympathetic  supply  to  the  heart. 

The  thoracic  or  dorsal  ganglia  give  rise  to  the  sympathetic 
supply  for  the  great  abdominal  viscera.  From  the  sixth,  seventh, 
eighth  and  ninth  springs  the  great  splanchnic  nerve,  which 
passes  through  the  diaphragm  to  the  semilunar  ganglion.  This 
is  the  largest  of  the  sympathetic  ganglion,  and  is  sometimes  called 
the  abdominal  brain.  It  has  been  inaccurately  called  the  center 
of  the  sympathetic  system.  The  two  ganglia  occupy  positions 
on  opposite  sides  of  the  celiac  axis,  and  give  rise  to  fibers  which 
supply  most  of  the  abdominal  viscera.  The  tenth  and  eleventh 
thoracic  ganglia  give  rise  to  the  lesser  splanchnic  nerve.  From 
the  last  thoracic  springs  the  renal  splanchnic  nerve.  The 
radiating  fibers  from  the  semilunar  ganglia  form  the  solar  plexuses 
for  the  two  sides. 

The  lumbar  ganglia  give  off  fibers  to  form  the  aortic  lumbar 
and  hypogastric  plexuses. 


THE   SYMPATHETIC   SYSTEM  299 

The  sacral  and  coccygeal  ganglia  supply  the  pelvic  vessels. 

Properties. — The  ganglia  and  nerves  are  slightly  sensitive. 
Contraction  of  involuntary  muscular  tissue  follows  stimulation — 
not  immediately,  but  after  a  considerable  interval,  and  the  sub- 
sequent relaxation  is  tardy.  Some  of  the  ganglia  are  dependent 
for  power  upon  their  fibers  from  the  cerebro-spinal  system,  while 
others  seem  capable  of  acting  independently,  at  least  for  a  time. 

Functions. — Little  is  known  of  the  functions  of  the  sympa- 
thetic except  as  regards  efferent  fibers.  They  are  distributed  in 
general  to  the  non-striped  musculature  of  the  circulatory  appara- 
tus and  of  the  viscera,  to  secreting  glands  and  to  the  heart.  The 
heart  furnishes  the  only  example  of  a  direct  sympathetic  supply 
to  striated  muscle.  The  sympathetic  has  a  very  definite  effect 
upon  secretion,  nutrition  and  the  local  production  of  heat.  Sec- 
tion of  the  sympathetic  fibers  going  to  any  part  causes  hyper- 
emia,  an  increased  amount  of  secretion  (sweat,  e.  #.),  and  a  rise 
of  temperature  in  that  part.  The  last  two  conditions  are  caused 
by  the  first,  and  it  in  turn  is  due  to  a  paralysis  of  the  muscular 
coat  of  the  vessels,  allowing  an  abrogation  of  their  usual  tonic 
condition  and,  consequently,  dilatation  and  an  increased  amount 
of  blood  with  exaggerated  nutritive  activity.  This  statement 
confronts  us  with  the  question  of  vaso-motor  action. 

Vaso-motor  Phenomena. — By  vaso-motor  nerves  is  meant 
those  fibers  which  convey  to  the  muscular  coat  of  the  vessel 
walls  impulses  causing  them  to  contract  and  decrease  the  caliber, 
or  to  relax  and  increase  it.  Those  causing  contraction  are  called 
vaso-constrictors ;  those  causing  relaxation  vaso-dilators.  It 
is  mainly  through  the  operation  of  vaso-motor  nerves  that  the 
sympathetic  system  influences  nutrition  in  a  particular  part, 
though  all  vaso-motor  fibers  are  not  confined  to  the  sympathetic 
cords.  However,  it  is  not  through  the  operation  of  the  vaso- 
motor  nerves  alone  that  the  sympathetic  lays  claim  to  be  the 
"system  of  nutrition,"  for  all  the  parts  to  which  its  other  fibers 
are  distributed  contribute  also  very  materially  to  nutrition, 


300  THE   NERVOUS    SYSTEM 

though  perhaps  in  not  so  direct  a  manner  as  do  the  muscular 
coats  of  the  arteries.  While  intestinal  peristalsis,  the  secretion 
of  many  glands,  as,  for  example,  the  production  of  glycogen,  bile, 
etc.,  cannot  be  shown  to  be  absolutely  dependent  on  sympathetic 
connections,  yet  all  these  processes — nutritive  in  nature — have 
their  normal  activity  seriously  impaired  by  withdrawal  of  the 
sympathetic  influence. 

The  chief  vaso-motor  center  is  in  the  medulla,  though  acces- 
sory centers  exist  also  in  the  cord;  all  vaso-motor  fibers  pass  out 
from  these  centers  and  leave  the  cerebro- spinal  axis  with  the 
cranial  or  spinal  nerves. 

The  most  usual  mode  of  action  of  the  vaso-motor  nerves  is 
reflex,  as  when  the  mucous  membrane  of  the  stomach  becomes 
hyperemic  upon  the  introduction  of  food;  or  when  the  salivary 
secretion  increases  during  mastication,  or  even  sometimes  at  the 
sight  or  thought  of  food;  or  when  emotions  are  evidenced  by 
paling  or  blushing. 

Raising  blood-pressure  by  stimulating  the  vaso-constrictors 
and  lowering  it  by  stimulating  the  vaso-dilators  are  simply 
mechanical  results,  and  require  no  comment. 

Sleep. — Sleep  is  closely  associated  with  vaso-motor  action. 
Every  part  of  the  body  has  a  function  to  perform,  but  it  must 
have  some  rest  from  that  performance  or  it  will  begin  to  act  in- 
efficiently and  finally  cease  altogether.  For  most  organs  these 
periods  of  rest  occur  at  approximately  uniform  intervals,  as  in 
case  of  the  stomach,  heart  or  respiratory  muscles;  but  notably 
in  case  of  the  involuntary  muscles  these  periods  of  repose  have 
no  regularity — i.  e.,  a  person  exercises  them  at  no  regular  time 
except  by  accident  of  occupation  or  otherwise.  But,  in  any  case, 
there  comes  a  time  when  repose  must  be  had,  for  during  activity 
the  destructive  processes  far  exceed  the  constructive,  and  in 
order  for  the  balance  to  be  preserved  there  must  be  a  time  when 
the  opposite  is  true. 


THE    SYMPATHETIC    SYSTEM  301 

Now  we  may  say  that  it  is  the  function  of  the  brain  to  furnish 
consciousness — if  we  can  allow  that  consciousness  embraces  all 
the  various  manifestations  of  nerve  force  peculiar  to  the  brain. 
For  the  brain  to  suspend  this  function  at  frequent  intervals  like 
the  heart  (e.  g.)  would  be  manifestly  impossible  if  one  is  to  do 
any  consecutive  work  depending  upon  this  organ.  The  brain 
works  longer,  and  must,  therefore,  rest  longer  at  a  time  than 
most  of  the  other  organs  of  the  body.  True,  so  far  as  the  volun- 
tary muscles  are  concerned  they  rest  best  probably  when  the 
brain  is  resting,  but  the  latter  condition  is  not  a  necessary  one 
for  the  maintenance  of  their  physiological  integrity.  This  repose 
of  the  brain — this  temporary  abolition  of  the  cerebral  functions — 
is  sleep.  While,  of  course,  the  activity  of  that  organ  during 
wakefulness  may  be  increased  or  diminished  by  volition,  and  it 
may,  therefore  rest  from  a  comparative  standpoint — as  when  one 
ceases  to  think  actively  upon  a  subject  and  becomes  mentally  list- 
less— still  the  brain  can  never,  under  such  circumstances,  rest 
properly,  and  sleep  finally  becomes  imperative. 

Vascular  Phenomena  of  Sleep. — Coma  is  analogous  to  sleep 
in  that  consciousness  is  lost;  but  in  this  case  the  brain  is  con- 
gested and  the  condition  is  unnatural.  It  was  long  supposed  that 
this  was  the  vascular  condition  during  natural  sleep,  but  appli- 
cation of  the  physiological  principles  prevailing  in  other  parts 
of  the  body  would  rather  presuppose  a  condition  of  cerebral 
anemia;  for  the  brain  receives  blood  for  two  purposes — first,  to 
supply  nutrition  to  the  nervous  substance,  and  second,  to  bring 
supplies  which,  by  the  action  of  the  brain  cells,  may  be  con- 
verted into  nerve  force — and  during  sleep  only  the  first  of  these 
purposes  is  to  be  served.  This  is  true  in  case  of  glands,  muscles, 
etc.,  during  their  intervals  of  repose.  As  a  matter  of  fact,  the 
cerebral  vessels  are  contracted  and  there  is  much  less  blood  in 
the  brain  during  sleep  than  during  consciousness. 

Dreams. — In  explanation  of  the  phenomena  of  dreams  and 
somnambulism,  it  is  said  that  what  we  call  sleep  may  occur  in 


302  THE   NERVOUS    SYSTEM 

one  part  of  the  brain  and  not  in  another,  or  in  different  degrees 
in  different  parts  of  the  nervous  centers.  "In  the  former  case 
[dreams]  the  cerebrum  is  still  partially  active;  but  the  mind 
products  of  its  action  are  no  longer  corrected  by  the  reception, 
on  the  part  of  the*  sleeping  sensorium,  of  impressions  of  objects 
belonging  to  the  outer  world;  neither  can  the  cerebrum,  in  this 
half-awake  condition,  act  on  the  centers  of  reflex  action  of  the 
voluntary  muscles,  so  as  to  cause  the  latter  to  contract — a  fact 
within  the  painful  experience  of  all  who  have  suffered  from  night- 
mare. In  somnambulism  the  cerebrum  is  capable  of  exciting 
that  train  of  reflex  nervous  action  which  is  necessary  for  pro- 
gression, while  the  nerve  center  of  muscular  sense  (in  the  cere- 
bellum?) is  presumably  fully  awake;  but  the  sensorium  is  still 
asleep,  and  impressions  made  on  it  are  not  sufficiently  felt  to 
rouse  the  cerebrum  to  a  comparison  of  the  difference  between 
mere  ideas  or  memories  and  sensations  derived  from  external 
objects"  (Kirkes). 

Relation  Between  the  Cerebro-spinal  and  Sympathetic 
Systems. — A  brief  resume  may  help  to  clarify  the  association 
between  the  two  systems. 

i.  Anatomically. — The  two  are  developed  from  the  same  em- 
bryological  tissue ;  the  vaso-motor  sympathetic  fibers  obey  centers 
in  the  medulla  and  cord,  and  must,  therefore,  be  connected  with 
those  centers  either  directly  or  indirectly;  characteristic  small 
medullated  fibers  pass  at  intervals  from  the  cord  through  the 
roots  into  the  sympathetic  ganglia;  they  send  fibers  each  to  the 
trunks  of  the  other  to  be  distributed  directly,  or  to  form  plexuses 
and  then  be  distributed  together;  their  fibers  are  found  together 
in  all  organs  which  receive  cerebro-spinal  nerves  (unless  they 
be  non-vascular) ;  in  some  of  these  organs  just  named  the  sym- 
pathetic fibers  are  there  only  as  vaso-motor  nerves,  while  in  others, 
as  glandular  structures  like  the  liver  and  salivary  glands,  sym- 
pathetic fibers  are  distributed  to  the  gland  cells  themselves, 
and  both  have  a  definite  but  associated  influence  on  secretion. 


THE   SYMPATHETIC   SYSTEM  303 

2.  Physiologically. — The  physiological  relation  is  best  indi- 
cated by  examples.  A  great  many,  if  not  all,  the  sympathetic 
ganglia  seem  to  receive  their  power  to  generate  nerve  force  from 
the  cerebro-spinal  system;  there  can  be  no  proper  nutrition  of  the 
parts  animated  by  cerebro-spinal  fibers  without  the  associated  ac- 
tion of  vaso-motor  sympathetic  fibers — not  even  of  the  nerve  cells 
and  fibers  themselves;  in  reflex  action  the  afferent  impression 
may  be  conveyed  by  a  cerebro-spinal  fiber  and  reflected  through 
a  sympathetic,  or  vice  versa;  when  one  hand  is  thrust  into  hot 
or  cold  water  the  temperature  of  the  opposite  hand  may  be  raised 
or  lowered,  impressions  having  been  carried  to  the  center  by 
cerebro-spinal  and  reflected  by  sympathetic  fibers,  not  only  to 
the  immersed  hand,  but  to  the  other  as  well;  food  is  taken 
into  the  mouth,  impressions  are  carried  by  nerves  of  common 
sensation  to  the  brain  and  are  reflected  through  the  sympathetic 
system,  an  increased  amount  of  blood  is  thereby  sent  to  the  sali- 
vary glands  and  an  increased  secretion  supervenes;  one  smells 
savory  articles  and  the  mouth  waters,  etc. 

Examples  could  be  multiplied  ad  infinitum  to  establish  the 
cooperation  existing  between  the  two  systems.  What  has  been 
incidentally  and  indirectly  said  on  this  point  in  considering  secre- 
tion, digestion,  circulation,  respiration,  etc.,  serves  to  emphasize 
their  connection. 


CHAPTER  XII. 

THE  SENSES. 

IT  is  evident  from  preceding  remarks  that  it  is  through  the 
intervention  of  the  nervous  system  that  we  have  a  "sense"  of 
existence,  of  the  existence  and  condition  of  different  parts  of 
our  bodies  and  of  our  relations  to  the  external  world.  The  knowl- 
edge we  thus  obtain  is  based  upon  sensations  of  various  kinds, 
all  of  which  are  carried  to  the  centers  by  afferent  fibers.  Such 
sensations  maybe  what  are  termed  (A)  Common,  or  (B)  Special, 
including  (i)  Touch,  (2)  Smell,  (3)  Sight,  (4)  Taste,  (5)  Hearing. 
It  is  to  be  remembered  that  the  seat  of  sensation  is  in  the  brain, 
and  not  in  any  organ  which  primarily  receives  or  conveys  the  im- 
pression. We  do  not  in  reality  see  with  the  eye  or  hear  with  the 
ear;  these  are  only  complex  organs  so  arranged  that  rays  of  light 
or  sound  waves  produce  upon  them  such  impressions  as,  when 
transmitted  to  the  sensorium,  will  give  rise  to  the  sensations  of 
sight  or  hearing. 

(A)  COMMON  SENSATIONS. 

As  regards  the  uses  of  the  fibers  conveying  impressions  which 
result  in  these  sensations,  they  (unless  it  be  those  concerned  with 
tactile  impressions)  are  distinct  from  those  of  special  sense. 
That  is  to  say,  the  fibers  of  the  olfactory,  optic,  gustatory  and 
auditory  nerves  do  not  convey  general  impressions;  but  it  is  al- 
most certain  that  fibers  conveying  tactile  impressions  convey 
also  painful  impressions — and  the  sensation  of  pain  is  taken  as 
typical  of  common  sensations.  It  is  known  that  very  painful 
impressions  sometimes  overcome  tactile  sensibility,  and  that 

304 


THE    SENSE    OF    TOUCH  305 

very  frequently  tactile  sensibility  remains  in  parts  which  receive 
no  painful  impressions,  as,  e.  g.,  under  anesthesia  by  cocain;  but 
it  may  be  that  the  power  in  the  same  fiber  to  convey,  in  the  first 
case  tactile,  and  in  the  second  painful  impressions  is  destroyed 
without  destroying  its  power  to  convey  the  other. 

The  varieties  of  common  sensation  are  too  numerous  to  even 
mention.  Thirst,  hunger,  fatigue,  discomfort,  satiety,  etc.,  are 
everyday  examples,  as  are  also  the  desire  to  urinate  or  defecate. 
Numerous  subdivisions  of  the  sensation  of  pain  might  be  men- 
tioned, such  as  itching,  burning,  aching,  etc.  The  so-called 
muscular  sense — by  which  we  become  aware  of  the  condition, 
relation,  coordination  and  degree  of  activity  or  repose  of  the 
muscles — will  be  considered  as  belonging  here. 

(B)  SPECIAL  SENSATIONS, 
i.  The  Sense  of  Touch. 

The  sense  of  touch  is  closely  related  to  common  sensation. 
Its  distribution  over  the  body  is  as  uniform  as  that  of  common 
sensation,  but  it  is  most  highly  developed  in  those  parts  where 
general  sensibility  is  most  marked  (as  in  the  skin),  and  attains 
its  highest  degree  of  perfection  only  in  those  situations  in  which 
tactile  corpuscles  exist,  for  example,  on  the  palmar  surfaces  of 
the  tips  of  the  fingers.  The  teeth,  hair,  nails,  etc.,  are  rather 
surprisingly  endowed  with  tactile  sensibility.  Leaving  pain  and 
the  muscular  sense  as  part  of  general  sensibility,  the  sense  of 
touch  may  be  considered  under  two  heads — (a)  Tactile  Sensi- 
bility proper  and  (b)  Temperature. 

(a)  Tactile  sensibility  proper  is  most  marked  where  the 
epidermis  over  the  papillae  is  thin.  When  the  epidermis  is  re- 
moved and  the  cutis  is  touched  there  is  pain  instead.  Tactile 
sensibility  is  much  decreased  where  the  epidermis  is  thickened, 
as  over  the  heel.  The  terminal  tactile  organs  have  been  de- 
scribed in  connection  with  afferent  nerves.  They  are  chiefly 

20 


306  THE   SENSES 

the  end  bulbs  of  Krause  and  the  tactile  corpuscles  of  Meiss- 
ner.  (See  Figs.  71  and  72.)  Besides,  tactile  impressions  are  re- 
ceived by  the  free  extremities  of  afferent  nerves  situated  over 
the  body  at  large.  Numbness  from  cold  is  due  to  interference 
with  cutaneous  circulation — upon  which  the  sense  of  touch  is 
directly  dependent.  It  is  almost  impossible  to  distinguish  mere 
touch  from  pressure. 

Acuteness. — How  the  sense  of  touch  is  capable  of  develop- 
ment by  practice  is  well  illustrated  in  the  case  of  many  blind 
persons.  They  learn  to  read  with 'comparative  facility  by  pass- 
ing the  hand  over  raised  letters;  or  they  frequently  make  the 
sense  of  touch  take  the  place  of  the  lost  sense  in  other  almost 
incredible  ways.  The  acuteness  of  this  sense  in  different  por- 
tions of  the  body  has  been  made  the  subject  of  observation  by 
touching  two  different  parts  in  the  same  region  with  finely  pointed 
instruments  and  noting  how  near  the  points  can  be  brought 
together  and  still  be  recognized  as  two.  This  distance  is  found 
to  vary  from  -^  inch  on  the  tip  of  the  tongue  to  i\  inches  in  the 
dorsal  region. 

(b)  It  is  not  improbable  that  there  are  special  nerve  endings 
concerned  in  the  reception  of  temperature  impressions,  though 
this  has  not  been  definitely  proven.  Decisions  as  to  tempera- 
ture are  only  relative;  the  surface  temperature  of  the  part  upon 
which  the  impression  is  made  is  the  standard,  and  one  can  only 
tell  absolutely  whether  the  object  is  hotter  or  colder  than  the 
skin,  and,  within  certain  limits,  approximate  how  much  hotter  or 
colder.  The  delicacy  of  the  temperature  sense  agrees  with 
that  of  touch  as  regards  the  thickness  or  absence  of  the  epidermis. 

2.  The  Sense  of  Smell. 

Regarding  the  mechanism  of  olfaction  it  is  found  that  one  of 
the  first  conditions  necessary  is  the  presence  of  particular  cells. 
Between  the  epithelial  cells  of  the  mucous  membrane  to  which 
the  olfactory  fibers  are  distributed  are  delicate  spindle-shaped 


THE   SENSE    OF    SIGHT  307 

cells  known  as  olfactory  cells,  and  to  them  pass  the  terminal 
filaments  from  the  olfactory  bulbs.  These  cells  are  stimulated 
by  contact  with  odorous  substances,  and  from  them  go,  by  way 
of  the  nerve  fibers,  impressions  which  are  recognized  as  odors 
of  different  kinds.  The  olfactory  fibers  are  the  only  ones  which 
will  convey  such  impressions.  True,  the  same  substance  may, 
at  the  same  time,  excite  other  sensations,  as  of  pain  or  taste, 
but  the  impressions  giving  rise  to  these  latter  sensations  are  con- 
veyed by  different  fibers  altogether.  The  substances  which  ex- 
cite olfaction  must  come  in  actual  contact  with  the  nerve  terminals 
and  to  do  this  must  be  dissolved  in  the  mucus  of  the  nasal 
mucous  membrane;  hence  dryness  of  the  nasal  cavities  (as  in  the 
first  stage  of  nasal  catarrh)  interferes  with  olfaction.  It  is  also 
said  that  odorous  substances  introduced  in  solution  into  the 
nasal  cavities  will  not  excite  the  sense  of  smell,  but  that  they 
must  be  introduced  by  a  current  of  air. 

Whether  an  odor  is  pleasent  or  unpleasant  is  largely  a  relative 
matter;  odors  most  disgusting  to  some  animals  are  not  offensive 
to  others.  This  same  difference  may  also  hold  good  among 
different  men.  Impairment  of  the  sense  of  taste,  for  some  reason, 
follows  a  loss  of  the  sense  of  smell. 

3.  The  Sense  of  Sight. 

It  is  not  intended  to  go  into  a  detailed  consideration  of  the 
sense  of  sight,  but  some  remarks  on  the  normal  eye  and  its  action 
are  in  order. 

Protection  of  the  Ball.— The  orbital  cavity  has  a  pyramidal 
shape  with  its  base  forward.  It  contains  the  eye-ball,  its  mus- 
cles, some  adipose  tissue  and  most  of  the  lachrymal  apparatus. 
Above  the  orbit,  the  eye-brows  prevent  a  flow  of  perspiration 
from  the  forehead  on  to  the  lid,  and  also  shade  the  eye  to  some 
extent.  The  lids,  when  closed,  entirely  obscure  the  balls  and 
protect  them  in  front.  On  their  free  borders  are  rows  of  hairs 
(eye-lashes)  curling  away  from  the  globe  and  shading  and  pro- 


308  THE    SENSES 

tecting  it  from  dust.  The  lids  are  closed  by  the  orbiculares  pal- 
pebrarum  and  opened  by  the  levatores  palpebrarum  superiores. 
In  the  ordinary  closing  of  the  lids  only  the  upper  one  is  moved, 
but  the  lower  one  is  raised  in  forcible  contraction  of  the  orbicu- 
laris.  Intervention  of  the  will  is  not  necessary  to  the  action  of 
these  muscles,  though  they  are  striated.  Except  during  fatigue, 
the  eyes  are  kept  open  involuntarily,  but  when  the  cornea  is 
touched  no  effort  of  the  will  can  prevent  contraction  of  the  or  icu- 
laris  palpebrarum.  During  sleep  the  globes  are  rotated  upward. 

The  Lachrymal  Apparatus.— This  consists  of  the  lachry- 
mal glands,  canal,  duct  and  sac,  and  the  nasal  duct.  The 
secretion  of  the  lachrymal  gland  keeps  the  cornea  and  conjunc- 
tiva constantly  bathed  in  a  thin  fluid.  It  is  situated  in  the  orbital 
cavity  at  its  upper  and  outer  portion.  Its  secretion  is  discharged 
upon  the  conjunctiva  by  several  little  ducts.  The  excess  of  se- 
cretion is  carried  into  the  riose  through  the  nasal  duct.  Near 
the  inner  canthus  is  a  small  opening  in  each  lid;  these  openings 
are  the  orifices  of  the  lachrymal  canals,  which  canals  join  at  the 
inner  angle  of  the  eye  to  form  the  lachrymal  sac ;  the  sac  is  con- 
tinued below  as  the  nasal  duct,  opening  into  the  inferior  meatus 
of  the  nose.  The  secretion  of  tears  is  much  diminished  during 
sleep.  The  influence  of  the  nervous  system  on  lachrymal  secre- 
tion is  well  known.  Emotional  disturbances  operate  through 
the  sympathetic  to  increase  the  flow.  Irritation  of  the  mucous 
membrane  of  the  nose  or  eye  is  followed  by  a  like  result. 

Movements  of  the  Ball.— The  capsule  of  Tenon,  a  fibrous 
membrane  outside  the  sclerotic,  holds  the  ball  loosely  in  place. 
A  small  amount  of  adipose  tissue  behind  the  globe  is  never  ab- 
sent. Movements  of  the  ball  are  effected  through  the  action 
of  the  internal  and  external  recti,  the  superior  and  inferior 
recti,  and  the  superior  and  inferior  oblique ;  of  these,  all  but 
the  two  last  named  arise  from  the  apex  of  the  orbital  cavity. 
The  recti  are  inserted  into  the  sclerotic  just  back  of  the  cornea. 
The  superior  oblique  runs  along  the  inner  aspect  of  the  orbital 


MOVEMENTS    OF   THE    BALL 


309 


cavity  to  a  point  near  the  supero-internal  angle;  here  it  becomes 
tendinous,  passes  through  a  nbro-cartilaginous  ring,  and  then 
turns  backward  and  outward  to  be  inserted  into  the  sclerotic 
between  the  superior  and  external  recti  just  behind  the  center 
of  the  globe.  The  inferior  oblique  arises  just  within  the  orbital 


FIG.  88. — Lateral  view  of  the  muscles  of  the  eye-ball. 

6,  optic  net 
(Landois.) 


5,  trochlea  or  pulley  of  the  superior  oblique  muscle,  12 ;  6,  optic  nerve;  8,  superior, 
9,  inferior,  and  12,  external  rectus;  13,  inferior  oblique. 


cavity  near  the  anterior  inferior  angle,  and  passes  around  the 
anterior  part  of  the  globe  to  be  inserted  in  the  sclerotic  just  below 
the  superior  oblique. 

The  effect  these  muscles  have  upon  the  movements  of  the  ball 
is  indicated  by  their  origin  and  attachment.  The  external  and 
internal  recti  rotate  it  outward  and  inward,  the  superior  and 
inferior  recti  upward  and  downward.  The  superior  and  inferior 
oblique  antagonize  each  other.  The  former  rotates  the  globe  so 


310  THE    SENSES 

that  the  pupil  is  directed  outward  and  downward;  the  latter  so 
that  it  looks  outward  and  upward.  The  associated  action  of  all 
these  muscles  can  produce  almost  any  variety  of  movements,  and 
no  effort  of  the  will  is  necessary  to  properly  associate  them  when 
it  is  desired  to  direct  the  line  of  vision  toward  a  certain  object. 
For  instance,  when  it  is  desired  to  look  at  an  object  on  the  right  it 
takes  no  distinct  voluntary  effort  to  contract  the  external  rectus  of 
the  right  eye  and  the  internal  rectus  of  the  left.  It  will  be  seen 
later  that  vision  for  the  two  eyes  is  normal  only  when  impressions 
are  made  upon  exactly  corresponding  parts  of  the  two  retinae,  so 
that  they  may  act  as  a  single  organ;  and  for  this  to  be  done  not 
always  the  same  movements  are  called  for  in  both  balls. 

Anatomy  of  the  Ball. — The  eye-ball  is  a  globular  body  con- 
sisting of  several  coats  enclosing  refracting  media.  Of  these 
coats  the  external  is  the  sclerotic,  dense  and  fibrous,  covering 
the  posterior  five-sixths  of  the  organ  and  continuous  with  the 
cornea,  which  covers  the  anterior  one-sixth.  It  is  not  well  sup- 
plied with  blood-vessels.  The  cornea  is  transparent,  and  upon 
its  external  surface  are  several  layers  of  delicate  nucleated  epithe- 
lium; underneath  this  layer  of  cells  is  a  thin  membrane,  the  an- 
terior elastic  lamella,  which  is  a  continuation  of  the  conjunc- 
tiva. The  substance  proper  of  the  cornea  is  composed  of  pale 
interlacing  fibers  among  which  are  connective  tissue  corpuscles 
and  quite  a  quantity  of  fluid.  These  fibers  are  continuous  from 
the  sclerotic,  but  they  lose  their  opacity  at  the  corneo-sclerotic 
margin.  On  the  posterior  surface  of  the  cornea  is  the  trans- 
parent elastic  membrane  of  Descemet,  a  part  of  which,  at  the 
circumference  of  the  iris,  passes  into  the  ciliary  muscle.  The 
cornea  is  very  sensitive,  but  contains  no  blood-vessels. 

Next  inside  the  sclerotic  is  the  choroid  coat  of  the  eye.  It 
does  not  lie  under  the  cornea,  but  is  confined  to  the  sclerotic 
area  of  the  ball.  Behind  the  optic  nerve  penetrates  it,  and  in 
front  it  is  connected  with  the  iris.  The  choroid  is  very  vascular. 
Its  color  is  dark  brown  on  account  of  the  abundance  of  pigment 


ANATOMY   OF    THE   BALL  311 

in  the  cells  on  the  inner  surface  of  the  membrane.  Anteriorly 
the  choroid  is  folded  in  upon  itself  to  form  the  ciliary  processes, 
which  project  inward  around  the  margin  of  the  crystalline  lens. 
The  ciliary  muscle  is  important  in  accommodation.  It  is  in 
the  shape  of  a  muscular  ring  surrounding  the  margin  of  the 
choroid  just  outside  the  ciliary  processes.  In  front  it  is  attached 


FIG.  89. — Diagram  of  a  vertical  section  of  the  eye.     (From  Yeo  after  Holden.) 

i,  anterior  chamber  filled  with  aqueous  humor;  2,  posterior  chamber;  3,  canal  of 
Petit;  a,  hyaloid  membrane;  b,  retina  (dotted  line);  c,  choroid  coat  (black  line);  d, 
sclerotic  coat;  e,  cornea;  f,  iris;  g,  ciliary  processes;  h,  canal  of  Schlemm  or  Pontana; 
i,  ciliary  muscle. 

to  the  line  of  junction  of  the  cornea  and  sclerotic  and  to  the  liga- 
ment on  the  anterior  surface  of  the  iris;  behind  it  is  lost  in  the 
substance  of  the  choroid.  Its  contraction,  therefore,  compresses 
the  vitreous  humor  and  relaxes  the  suspensory  ligament  of  the 
lens.  The  iris  is  a  circular  veil  hanging  in  front  of  the  lens.  It 
presents  a  perforation  a  little  to  the  nasal  side  of  its  center,  the 
pupil.  It  is  attached  in  the  corneo-sclerotic  line.  It  contains 
circular  and  radiating  fibers.  The  iris  divides  the  space  between 
the  cornea  and  lens  into  two  chambers,  anterior  and  posterior 


312  THE    SENSES 

— the  latter  of  which  is  very  small.  The  " color  of  the  eyes" 
depends  on  the  color  of  the  anterior  surface  of  the  iris;  its  pos- 
terior surface  has  a  constant  dark  purple  hue.  The  size  of  the 
pupil  is  subject  to  variations  to  be  noted  later. 

Inside  the  choroid  is  the  retina,  which  is  that  part  of  the  eye 
capable  of  receiving  impressions  of  sight.  Anteriorly  it  reaches 
nearly  to  the  ciliary  processes.  Externally  it  is  in  contact  with 
the  choroid,  and  internally  with  the  hyaloid  membrane  of  the 
vitreous  humor.  It  is  penetrated  by  the  optic  nerve  a  little  within 
and  below  the  center  of  the  posterior  hemisphere.  Just  external 
to  the  point  of  entrance  of  the  nerve  is  the  macula  lutea,  a  small 
yellow  area  in  the  center  of  which  is  the  fovea  centralis ;  this 
last  is  exactly  in  the  axis  of  distinct  vision.  Nine  layers  of  cells 
are  usually  described  as  composing  the  retina.  From  without 
inward  they  are  (i)  the  pigment  layer,  (2)  rods  and  cones,  (3-6)  the 
four  granular  layers,  (7)  nerve  cells,  (8)  expansion  of  fibers  of  the 
optic  nerve,  (9)  the  limitary  membrane.  Of  these,  the  most  im- 
portant is  the  layer  of  rods  and  cones.  The  rods,  or  cylinders, 
extend  through  the  thickness  of  the  membrane  and  have  between 
them,  at  intervals,  flask- shaped  bodies,  the  cones.  At  the  macula 
lutea  only  the  cones  exist.  Elsewhere  the  rods  are  more  abun- 
dant than  the  cones.  The  length  of  the  cones  is  about  half  that 
of  the  rods,  and  they  occupy  the  inner  aspect  of  the  membrane. 
The  layer  of  nerve  cells  presents  cells  communicating  on  the 
one  hand  with  the  rods  and  cones  and  on  the  other  with  fibers 
of  the  optic  nerve.  The  rods  and  cones  are  the  only  parts  of 
the  retina  possessing  special  sensibility,  impressions  being  con- 
veyed from  them  to  the  brain  by  the  optic  nerve.  The  fibers 
of  the  second  nerve,  composing  one  layer,  are  pale  and  transpar- 
ent. The  blood  supply  of  the  retina  is  from  the  arteria  cen- 
tralis retinae,  which  enters  the  optic  nerve  just  before  it  expands, 
and,  running  in  its  substance,  is  distributed  as  far  as  the  ciliary 
processes  anteriorly. 

The  Crystalline  Lens  is  a  biconvex  transparent  body  situ- 


OCULAR   REFRACTION  313 

ated  just  behind  the  iris.  Its  function  is  to  refract  the  rays  of 
light,  and  its  action  in  this  respect  is  similar  to  such  lenses  in 
optical  instruments.  It  is  held  in  place  by  the  suspensory  liga- 
ment. Its  anterior  convexity  is  more  marked  than  its  posterior. 
It  is  enveloped  by  a  thin  transparent  capsule. 

The  Suspensory  Ligament  is  a  continuation  of  the  anterior 
layer  of  the  hyaloid  membrane  of  the  vitreous  humor.  When 
this  layer  reaches  the  edge  of  the  lens  (coming  forward)  it  divides 
into  two  parts,  one  passing  in  front  of  and  the  other  behind  that 
body;  the  divisions  are  continuous  respectively  with  the  anterior 
and  posterior  portions  of  the  capsule  of  the  lens.  The  ligament 
supports  the  lens. 

The  Aqueous  Humor  is  behind  the  cornea  and  in  front  of 
the  lens  and  suspensory  ligament.  The  iris  has  been  said  to 
separate  this  cavity  into  anterior  and  posterior  chambers  com- 
municating through  the  pupillary  opening.  The  aqueous  hu- 
mor is  colorless  and  perfectly  transparent.  It  serves  to  refract 
the  rays  of  light,  having  for  that  purpose  the  same  index  as  the 
cornea. 

The  Vitreous  Humor  occupies  about  the  posterior  two-thirds 
of  the  globe,  and  is  back  of  the  lens  and  suspensory  ligament 
surrounded  by  the  delicate  hyaloid  membrane.  It  is  of  a  gelat- 
inous consistence,  and  is  divided  into  numerous  compartments 
by  very  delicate  membranes  radiating  from  the  point  of  entrance 
of  the  optic  nerve.  It  is  a  transparent  refracting  medium. 

Ocular  Refraction. — In  order  for  the  image  of  an  object  to  be 
distinct  the  rays  passing  from  it  must  fall  on  a  single  portion  of 
the  retina,  viz.,  the  fovea  centralis.  The  sensibility  of  the 
retina  to  light  decreases  in  passing  away  from  the  fovea.  All 
rays  would  not  meet  on  the  retina  unless  they  were  refracted; 
and  for  this  purpose  there  are  the  cornea,  the  aqueous  humor, 
the  lens  and  the  vitreous  humor.  The  surfaces  of  the  cornea 
and  lens  are  the  most  important  of  these.  Since  the  two  surfaces 
of  the  cornea  are  parallel,  the  external  surface  alone  is  concerned 


314  THE   SENSES 

in  refraction.  The  center  of  distinct  vision  (fovea)  is  in  the  axis 
of  the  lens  precisely  in  the  plane  upon  which  the  rays  of  light 
are  brought  to  a  focus  by  the  refracting  media.  Refraction  by 
the  cornea  alone  would  focus  the  rays  behind  the  retina;  hence  the 
necessity  of  convex  lenses  before  the  eye  after  operations  for 
cataract.  Rays  leaving  the  cornea  are  refracted  by  the  anterior 
surface  of  the  lens,  by  its  substance  to  a  certain  extent,  and  again 
by  its  posterior  surface,  the  normal  mechanism  being  such  that 
all  rays  are  focused  on  the  fovea.  The  rays  cross  each  other  after 
refraction,  and  the  image  is  inverted,  but  the  brain  takes  no 
notice  of  this  fact,  and  objects  are  seen  in  their  natural  positions. 

Accommodation. — Accommodation  means  a  change  in  the 
convexity  of  the  lens,  whereby  images  are  focused  on  the  retina, 
whether  the  object  be  far  away  from  or  near  the  eye.  Rays  of 
light  from  distant  objects  strike  the  eye  practically  parallel,  and 
we  may  assume  that  there  is  a  certain  "passive"  condition  of 
the  refracting  media  which  will  bring  such  rays  to  a  focus  at  the 
proper  point.  But  when  the  object  observed  is  near  the  eye  a 
change  in  the  arrangement  of  the  media,  or  of  the  convexity  of 
their  surfaces,  is  necessary  to  prevent  the  focusing  of  the  rays 
behind  the  retina.  The  desired  end  is  accomplished  by  increas- 
ing the  convexity  of  the  lens.  When  the  ciliary  muscle  is  "pas- 
sive" the  capsule  compresses  the  lens,  decreasing  its  convexity 
to  a  minimum;  from  the  attachments  of  this  muscle,  already 
noted,  its  contraction  is  attended  by  a  relaxation  of  the  suspensory 
ligament,  which  in  turn  relieves  in  some  degree  the  compression 
of  the  capsule  upon  the  lens  and  allows  its  antero-posterior  diam- 
eter to  increase;  the  result  is  increased  convexity  of  the  lens. 

When  distant  objects  are  looked  at  the  lens  become  flatter  as 
a  result  of  contraction  of  the  suspensory  ligament,  which  contrac- 
tion is  a  consequence  of  the  relaxation  of  the  ciliary  muscle. 
Accommodation  for  distant  objects  seems  a  passive  process 
entirely. 

The  ciliary  muscle  is  the  "muscle  of  accommodation." 


REACTION    TO    LIGHT  315 

The  contraction  of  the  pupil  for  near  objects  is  not,  properly 
speaking,  a  part  of  accommodation. 

Then,  granting  special  sensibility  to  the  retina  and  optic  nerve 
the  formation  and  appreciation  of  an  image  is  simple.  Rays  of 
light  having  passed  through  the  cornea  and  aqueous  humor  are 
admitted  by  the  pupil  to  pass  through  the  lens  and  vitreous  hu- 
mor. By  all  these  objects  they  are  refracted  so  that  they  cross 
each  other  and  fall  upon  the  retina,  producing  an  inverted  image 
there.  The  size  of  the  pupil,  other  things  being  equal,  is  regu- 
lated by  the  intensity  of  the  light,  the  opening  being  contracted  to 
admit  less  when  the  light  is  strong. 

Myopia,  Hyperopia  and  Presbyopia. — Sometimes  the  antero- 
posterior  diameter  of  the  eye-ball  is  too  long  and  the  rays  of 
light  are  brought  to  a  focus  in  front  of  the  retina.  Such  a  con- 
dition is  known  as  myopia;  the  person  will  be  near-sighted. 
He  brings  objects  near  his  eyes  so  that  the  rays  may  have  a  greater 
divergence  and  thus  be  focused  farther  back.  Or  the  rays  may 
be  scattered  by  placing  concave  lenses  before  the  eyes.  Some- 
times, too,  the  antero-posterior  diameter  may  be  too  short  and 
the  rays  come  to  a  focus  behind  the  retina.  Such  a  condition  is 
known  as  hyperopia ;  the  person  will  be  far-sighted.  He  holds 
objects  far  away  from  his  eyes  that  the  rays  from  them  may  strike 
the  ball  with  less  divergence  and  thus  be  focused  farther  forward. 
Or  the  same  end  may  be  accomplished  by  placing  convex  lenses 
before  the  eyes.  In  old  age  the  lens  becomes  flattened  and  accom- 
modates itself  less  easily.  This  tends  to  focus  light  behind  the 
retina  and  objects  have  to  be  held  far  away  from  the  eye.  This 
is  known  as  presbyopia.  Its  remedy  is  the  same  as  that  for 
hyperopia. 

Reaction  to  Light. — Regarding  the  reaction  of  the  pupil  to 
light,  it  is  evident  that  this  is  mainly  a  reflex  nervous  phenom- 
enon, though  direct  light  will  cause  the  muscular  tissue  of  the 
iris  to  contract.  The  direct  influence  of  the  third  nerve  on  the 
action  of  the  iris  has  been  referred  to  under  a  consideration  of 


316  THE    SENSES 

that  nerve.  Reflexly,  the  pupil  is  contracted  by  light  by  the 
conveyance  of  an  impression  to  the  brain  through  the  optic  fibers, 
a  message  is  sent  to  the  proper  center,  and  a  stimulus  is  reflected 
through  the  third  nerve  to  the  sphincter  of  the  iris  causing  it  to 
contract.  When  the  optic  nerve  is  cut  the  circuit  is  broken,  and 
movements  of  the  iris  do  not  occur  from  the  admission  of  light. 
Practically,  then,  when  much  or  little  light  reaches  the  retina 
the  pupil  contracts  or  dilates,  as  the  case  may  be,  in  an  effort  to 
keep  the  amount  constant. 

Binocular  Vision. — It  is  evident  that  when  a  person  looks  at 
an  object  two  images  are  formed — one  on  each  retina — but  they 
are  combined  in  his  consciousness  and  he  sees  but  one  object. 
If  one  of  the  balls  be  thrown  out  of  the  proper  axis,  by  pressure, 
e.  g.,  objects  appear  double.  The  same  is  true  in  strabismus,  at 
least  until  the  person  has  grown  accustomed  to  the  defect.  In 
normal  vision  the  rays  from  an  object  are  formed  on  the  fovea 
centralis  of  each  eye — that  is,  upon  corresponding  points  which 
are,  for  each,  the  centers  of  distinct  vision. 

4.  The  Sense  of  Taste. 

In  order  that  gustatory  sensation  may  be  exercised  it  is  neces- 
sary (i)  that  there  be  specially  endowed  nerves  and  nerve  centers; 
(2)  that  the  nerve  terminals  be  excited  by  sapid  (tastable)  ma- 
terials; (3)  that  these  substances  be  in  solution.  It  has  already 
been  seen  that  the  special  nerves  of  taste  are  (a)  the  chorda  tym- 
pani  distributed  to  the  anterior  two-thirds  of  the  tongue,  and  (&) 
the  glosso-pharyngeal  to  the  posterior  third  of  that  organ.  It 
is  probable  that  only  the  dorsum  of  the  tongue,  the  lateral  parts 
of  the  soft  palate,  the  uvula  and  the  upper  pharynx  are  concerned 
in  gustation.  On  the  tongue  are  found  special  papillae,  (i)  the 
circumvallate,  large  and  few  in  number,  near  the  base  of  the 
organ,  and  (2)  the  fungiform,  about  200  in  number,  over  the 
remaining  area.  The  circumvallate  and  some  of  the  fungiform 


THE    SENSE    OF   HEARING  317 

papillae  contain  taste  beakers,  true  gustatory  organs.  They 
are  ovoid  collections  of  cells  beneath  the  epithelial  covering  of 
the  mucous  membrane.  Sapid  substances  enter  these  beakers 
in  solution  and  come  in  contact  with  the  taste  cells,  which  are 
connected  with  the  filaments  of  the  gustatory  nerves.  Thus 
are  produced  specific  impressions  which  are  conveyed  to  the 
gustatory  center,  and  the  sense  of  taste  is  excited.  The  limited 
distribution  of  the  taste  beakers  makes  it  impossible  that  they 
should  be  the  only  organs  capable  of  receiving  special  gustatory 
impressions.  The  taste  center  has  been  indefinitely  located  in 
the  uncinate  gyrus  near  the  olfactory  center. 

Since  it  is  necessary  to  the  tasting  of  substances  that  they 
come  in  actual  contact  with  the  taste  organs,  and  since  to  do  so 
they  must  be  in  solution,  it  follows  that  dryness  of  the  mouth 
interferes  with,  or  abolishes,  this  sense. 

The  most  marked  tastes  are  the  sweet,  bitter,  saline,  and  al- 
kaline. The  more  delicate  flavors  involve  also  the  special  sense 
of  smell,  and  it  has  been  seen  that  dissociation  of  the  two  kinds 
of  impressions  is  often  impossible.  Taste  is  also  subject  to 
variations  by  reason  of  education,  age,  association,  caprice,  etc. 
Bitters  are  most  easily  appreciated  at  the  back,  salts  and  sweets 
at  the  tip,  and  acids  at  the  sides  at  the  tongue. 

5.  The  Sense  of  Hearing. 

The  ear  consists  of  a  complicated  apparatus  for  the  purpose 
of  the  reception  of  special  impressions  which  are  appreciated  by 
the  brain  as  sounds.  Anatomically  it  consists  of  the  external, 
the  middle  and  the  internal  ear;  the  last  contains  the  essentials 
of  the  auditory  apparatus,  the  external  and  middle  divisions 
serving  only  to  concentrate  the  sound  waves  upon  the  parts  of 
the  internal. 

The  External  Ear.— This  consists  of  the  pinna  and  the  ex- 
ternal auditory  canal.  The  pinna  is  the  external  visible  por- 


318  THE    SENSES 

tion,  and  consists  of  the  large  cavity,  the  concha,  into  which  the 
external  auditory  canal  opens  externally;  of  two  prominent 
ridges  partly  surrounding  the  concha,  the  helix  outside  and  the 
antehelix  internal  to  this;  and  of  a  nbro-cartilaginous  process 
projecting  backward  in  front  of  the  concha,  the  tragus.  The 


FIG.  90. — Scheme  of  the  organ  of  hearing. 

AG,  external  auditory  meatus;  T,  tympanic  membrane;  K,  malleus  with  its  head 
(h) ,  short  process  (kf)  and  handle  (m) ;  a,  incus,  its  short  process  (x)  and  its  long 
process  united  to  the  stapes  (s)  by  means  of  the  Sylvian  ossicle  (Z);  P,  middle  ear; 
o,  fenestra  ovalis;  r,  fenestra  rotunda;  x,  beginning  of  the  lamina  spiralis  of  the 
cochlea;  pt,  scala  tympani,  and  vt,  scala  vestibuli;  V,  vestibule;  S,  saccule;  U, 
utricle;  H,  semicircular  canals;  TE,  Eustachian  tube.  The  long  arrow  indicates  the 
line  of  traction  of  the  tensor  tympani;  the  short  curved  one,  that  of  the  stapedius. 
(Landois.) 

external  auditory  canal  runs  inward  and  slightly  forward  from 
the  concha  to  terminate  at  the  membrana  tympani,  or  drum. 
Its  inner  part  is  in  the  petrous  portion  of  the  temporal  bone;  its 
external  part  is  nbro-cartilaginous  in  structure.  The  whole  is 
lined  by  integument. 

The  Middle  Ear  (Tympanum). — This  is  a  cavity  at  the 
bottom  of  the  external  auditory  canal  in  the  petrous  portion  of 


THE    INTERNAL    EAR  319 

the  temporal  bone,  containing  ossicles  for  the  conduction  of 
sound  waves  to  the  internal  ear.  The  cavity  communicates, 
through  the  Eustachian  tube,  with  the  pharynx,  and  this  is  its 
oniy  direct  connection  with  the  external  air,  though  it  does  com- 
municate with  the  mastoid  air  cells.  It  is  lined  by  mucous 
membrane.  The  membrana  tympani,  separating  it  from  the 
external  auditory  canal,  is  fibrous  in  structure.  It  is  lined  ex- 
ternally by  skin  and  internally  by  mucous  membrane. 

The  three  ossicles  of  the  middle  ear  are  the  malleus,  incus 
and  stapes.  The  malleus,  shaped  like  a  hammer,  is  attached 
in  a  vertical  direction  to  the  upper  radius  of  the  membrana  tym- 
pani, and  articulates  by  its  head  with  the  incus.  The  incus  has 
the  shape  of  an  anvil;  its  base  articulates  with  the  malleus, 
while  its  small  extremity  curves  downward  to  articulate  with  the 
neck  of  the  stapes.  The  base  of  the  stapes  is  applied  to  the 
membrane  covering  the  fenestra  ovalis.  The  tensor  and  laxa- 
tor  tympani  are  attached  to  the  neck  of  the  malleus;  the  sta- 
pedius  to  the  neck  of  the  stapes.  These  bones  constitute  a 
chain,  which  conveys  the  vibrations  of  the  membrana  tympani 
to  the  fenestra  ovalis. 

The  Internal  Ear  (Labyrinth). — This  consists  of  a  series 
of  cavities  in  the  petrous  portion  of  the  temporal  bone  lined  by 
a  peculiar  membrane.  When  the  bony  substance  surrounding 
these  cavities  is  carefully  removed  it  is  found  that  that  portion 
immediately  outside  them  is  harder  than  the  adjacent  structure. 
This  constitutes  the  bony  labyrinth,  while  the  membrane  inside 
the  bony  walls  is  the  membranous  labyrinth. 

The  bony  labyrinth  consists  of  the  vestibule,  cochlea  and 
semicircular  canals.  The  vestibule  occupies  the  mid-portion 
of  the  labyrinth,  and  is  that  part  with  which  the  middle  ear  com- 
municates by  the  fenestra  ovalis;  it  communicates  also  with  the 
cochlea  and  semicircular  canals,  and  on  its  internal  aspect  are 
openings  for  the  entrance  of  some  of  the  branches  of  the  audi- 
tory nerve.  The  cochlea,  shaped  like  a  snail  shell,  runs  off 


320 


THE    SENSES 


from  the  front  of  the  vestibule,  winds  about  two  and  a  half  times 
around  a  cone-shaped  central  axis — the  modiolus — and  ends 


FIG.  91. 

I,  Transverse  section  of  a  turn  of  the  cochlea;  II,  A,  ampulla  of  a  semicircular 
canal  with  the  crista  acustica;  a,  auditory  cells,  p,  provided  with  a  fine  hair;  T, 
otoliths;  III,  scheme  of  the  human  labyrinth;  IV,  scheme  of  a  bird's  labyrinth;  V, 
scheme  of  a  fish's  labyrinth.  (Landois.) 


in  a  blind  apex.     The  canal  of  the  cochlea  is  partially  separated 
into  two  compartments  by  a  bony  plate,  the  lamina  spiralis. 


TERMINATION   OF  AUDITORY  NERVE  321 

The  basilar  membrane  completes  the  septum  and  divides  the 
lumen  of  the  cochlea  into  two  canals,  the  scala  tympani  and 
the  scala  vestibuli,  corresponding  in  name  to  the  tympanic 
and  vestibular  openings  of  the  cochlea.  The  semicircular 
canals,  three  in  number — superior,  external  and  posterior — de- 
scribe arches  from  the  posterior  aspect  of  the  vestibule,  com- 
municating by  both  their  extremities  with  that  cavity. 

The  membranous  labyrinth  consists  of  a  special  membrane 
lying  inside  the  bony  labyrinth  and  corresponding  in  general 
outline  to  the  walls  of  the  cavity.  It  is,  however,  separated  from 
the  walls  by  perilymph,  and  encloses  a  similar  fluid,  the  en- 
dolymph.  It  covers  the  sides  of  the  lamina  spiralis  in  the  cochlea 
and  completes  the  septum,  besides  following  the  wall  proper; 
and  on  one  side  it  sends  a  distinct  process  from  the  tip  of  the 
a  mina  spiralis  to  the  wall  of  the  canal,  so  that  there  are  in  reality 
three  divisions  of  the  lumen  of  the  cochlea.  This  process  is  the 
membrane  of  Reissner,  and  the  third  canal  is  the  scala  media 
the  true  membranous  cochlea.  (See  Fig.  91.) 

Termination  of  Auditory  Nerve. — The  membranous  laby- 
rinth, containing  and  being  suspended  in  fluid,  receives  the  term- 
inal filaments  of  the  eighth  nerve  as  well  as  all  the  sonorous  vibra- 
tions intended  for  that  nerve.  When  the  auditory  nerve  has 
reached  the  base  of  the  internal  auditory  meatus  it  enters  the 
internal  ear  by  two  divisions,  one  for  the  vestibule  and  semicir- 
cular canals  and  the  other  for  the  cochlea.  The  vestibular 
portion  again  subdivides,  sending  one  branch  to  the  utricle  and 
superior  and  horizontal  semicircular  canals,  and  another  to  the 
saccule  and  posterior  semicircular  canal.  The  fibers  of  the 
eighth  nerve  spread  out  over  the  inner  surface  of  the  membrane 
to  end  in  a  way  somewhat  obscure.  The  membrane  is  lined 
internally  by  epithelium  whose  character  differs  in  different  areas. 
In  the  region  of  distribution  of  the  vestibular  portion  of  the  nerve 
the  cells  are  of  two  kinds,  hair  cells  and  rod  cells.  From  the 
inner  ends  of  the  hair  cells  ciliated  processes  project  into  the  en- 

21 


322  THE    SENSES 

dolymph;  to  their  outer  ends  pass  the  axis  cylinders  of  the  nerve 
fibers,  though  the  exact  mode  of  connection  is  not  clear.  The 
rod  cells  are  much  more  numerous  than  the  hair  cells,  but  their 
precise  connection  with  audition  is  not  apparent. 

Upon  the  basilar  membrane  are  the  rods  of  Corti.  They  con- 
sist of  two  sets  of  pillars  of  varying  length,  slanting  toward  each 
other,  thus  leaving  at  their  base  a  space  which  becomes  a  canal 
by  a  longitudinal  succession  of  these  pillars.  There  are  sup- 
posed to  be  about  4,500  elements  in  the  outer  and  6,500  in  the 
inner  set  of  these  rods.  Intimately  associated  with  the  pillars 
are  large  numbers  of  hair  cells  with  which  the  auditory  nerve 
filaments  may  communicate;  it  is  certain  that  these  filaments  are 
closely  connected  in  some  way  with  the  pillars. 

Functions  of  the  Semicircular  Canals. — The  use  of  these  is 
obscure.  Their  destruction  is  not  followed  by  interference  with 
hearing,  although  auditory  filaments  are  distributed  to  some 
parts  of  them.  Curiously  enough,  however,  this  lesion  is  one  of 
the  three  chief  ones  interfering  so  markedly  with  equilibrium — 
the  phenomena  following  it  being  not  unlike  those  sequent  upon 
lesions  of  the  cerebellum  and  the  posterior  white  columns  of  the 
cord. 

Functions  of  the  Cochlea. — While  the  exact  mechanism  of 
the  production  of  auditory  impressions  is  unknown,  there  seems 
to  be  no  doubt  that  such  mechanism  takes  place  almost  entirely 
in  the  cochlea,  and  that  fibers  which  convey  to  the  auditory  cen- 
ters impressions  of  sound  are  distributed  to  the  organ  of  Corti 
therein.  That  is  to  say,  loss  of  the  sense  of  hearing  supervenes 
upon  destruction  of  this  part  of  the  internal  ear.  In  physics  it 
is  known  that  for  a  sound,  for  example  of  a  piano  string,  to  be 
heard  the  membrana  tympani  must  vibrate  in  unison  with  the 
sonorous  vibrations  of  the  cord;  that  is,  "consonating  bodies" 
repeat  sonorous  vibrations,  giving  them  their  proper  pitch  and 
quality.  It  has  been  supposed  that  the  thousands  of  rods  of 
Corti,  of  varying  length  and  size,  in  the  Cochlea  are  made  to 


FUNCTIONS  OF  THE  COCHLEA  323 

vibrate  separately  or  in  correctly  associated  collections  (like  the 
strings  of  a  harp),  and  thus  reproduce  communicated  vibrations, 
and  so  give  rise  to  impressions  which,  conveyed  by  the  auditory 
nerve  to  the  center,  are  there  recognized  as  sounds  of  different 
degrees  of  intensity,  pitch  and  quality.  This  theory  may  be 
true,  but  its  correctness  is  probably  beyond  the  range  of  experi- 
mental proof. 

While  the  usual  mode  of  conduction  of  sound  waves  to  the 
cochlea  is  through  the  external  ear,  they  may  reach  it  in  other 
ways,  as  through  the  bones  of  the  head,  or  through  the  Eustach- 
ian  tube.  Nor  is  the  integrity  of  the  membrana  tympani  actually 
necessary  to  the  production  of  sound;  although  practically  speak- 
ing a  person  in  whom  this  organ  is  destroyed  is  deaf,  he  can  hear 
if  the  ossicles  can  in  some  way  be  placed  in  vibration  by  sound 
waves,  as  by  the  intervention  of  an  artificial  membrane.  In- 
deed it  has  already  been  seen  that  none  of  the  parts  of  the  ex- 
ternal or  middle  ear  are  actually  necessary  to  hearing.  They 
are  only  accessory  conveniences  for  the  better  t.ansmission  of 
impressions  to  the  filaments  of  the  auditory  nerve. 

The  (so-called)  tensor  and  laxator  tympani  muscles  make 
tense  or  lax  the  membrana  tympani,  thus  influencing  the  rapidity 
and  amplitude  of  its  vibrations,  and  therefore  the  pitch  and  in- 
tensity of  the  sound.  The  stapedius  prevents  too  great  move- 
ments of  the  stapes.  The  free  communication  of  the  air  in  the 
tympanum  with  that  in  the  mastoid  cells  and  pharynx  insures 
an  approximately  constant  internal  pressure  upon  the  membrane, 
and  thus  precludes  accidents  which  would  otherwise  interfere 
with  its  proper  vibration. 

The  auditory  center  in  man  is  in  the  first  and  second  temporal 
convolution  of  the  temporo-sphenoidal  lobe. 

Briefly  then,  the  physiology  of  hearing  is  as  follows:  Sound 
waves  collected  by  the  pinna  enter  the  external  auditory  canal 
and  impinge  upon  the  membrana  tympani.  The  drum  is  thus  set 
to  vibrating  and  communicates  its  movements  to  the  ossicles, 


324  THE    SENSES 

which  in  turn  hand  them  over  through  the  fenestra  ovalis  to  the 
fluids  of  the  internal  ear,  through  which  media  they  reach  the 
auditory  filaments,  are  conducted  to  the  brain  and  given  proper 
recognition. 

The  Production  of  the  Voice. 

The  production  of  the  voice  is  not  connected  with  the  special 
senses,  but  its  consideration  will  be  introduced  here  for  the  sake 
of  convenience. 

The  Larynx  is  the  organ  of  voice.  It  is  a  cavity  closed  ex- 
cept for  its  openings  above  and  below.  It  conisists  of  four  car- 
tilages— cricoid,  thyroid  and  two  arytenoid — joined  together  by 
ligaments  and  muscles.  The  vocal  cords  are  attached  poste- 
riorly to  the  bases  of  the  movable  arytenoid  cartilages  and  ante- 
riorly to  the  angle  between  the  alae  of  the  thyroid.  The  muscles 
serve  to  move  the  cartilages  and  thus  to  separate  or  approximate 
and  to  render  lax  or  tense  the  vocal  cords. 

Production  of  Sound. — The  human  voice  is  produced  by 
vibrations  of  the  vocal  cords,  which  vibrations  are  set  up  by 
currents  of  expired  air. 

Movements  of  the  Vocal  Cords. — These  are  those  taking 
place  (i)  in  respiration  and  (2)  during  vocalization. 
j,  i.  In  Respiration. — When  the  cords  are  "passive"  they  are 
approximated  anteriorly  and  separated  posteriorly,  so  that 
the  interval  between  them  (rima  glottidis)  is  triangular.  This 
interval  becomes  a  little  wider  during  inspiration  and  a  little 
narrower  during  expiration. 

2.  In  Vocalization. — The  production  of  sound  in  the  larynx 
involves  an  approximation  of  the  cords  and  an  increase  in  their 
tension.  They  are  made  more  nearly  parallel  by  the  approach 
of  the  arytenoids  to  each  other,  and  the  rima  glottidis  assumes 
the  shape  of  a  mere  chink.  The  tenser  the  cords,  the  higher  the 
note  produced;  usually  also  the  closer  the  cords  are  brought  to- 


THE   PRODUCTION   OF   THE   VOICE  325 

gether,  the  higher  the  note.  The  range  of  the  voice  depends 
principally  on  the  degree  of  tension  which  the  cord  can  be  made 
to  assume. 

Varieties  of  Vocal  Sounds. — These  are  mainly  (i)  monoto- 
nous, (2)  transitional,  (3)  musical. 

1.  In  monotonous  sounds  the  notes  have  all  nearly  the  same 
pitch,   as  in   reading. 

2.  In  transitional  sounds  there  is  a  gradual  change  in  the 
tension  and  approximation  of  the  cords,  so  that  the  notes  become 
successively  higher  or  lower,  as  in  the  howling  of  a  dog. 

3.  In  musical  sounds  the  vocal  cords  have  a  definite  number 
of  vibrations  for  each  successive  note — a  number  corresponding 
to  the  production  of  that  note  in  the  musical  scale. 

The  range  of  the  average  human  voice  is  from  one  to  three 
octaves.  The  highest  and  lowest  notes  of  females  are  about  one 
octave  higher  than  the  corresponding  notes  of  males.  The  chief 
difference  between  male  and  female  voices  is,  therefore,  one  of 
pitch;  but  they  also  differ  materially  in  tone.  The  difference 
in  pitch  is  a  result  of  the  different  length,  and  therefore  the  dif- 
ferent rate  of  vibration,  of  the  cords  in  the  two  sexes.  The 
female  cords  are  about  two-thirds  the  length  of  the  male. 

Before  puberty  the  male  larynx  resembles  the  female,  but  at 
that  period  the  alae  of  the  thyroid  becomes  more  prominent  in 
the  male  and  the  cords  increase  in  length,  thus  accounting  for 
the  change  of  voice. 

In  old  age  control  of  the  musculature  of  the  larynx  is  partly 
lost,  the  cords  become  altered  and  the  cartilages  ossify.  These 
circumstances  make  the  voice  weak  and  unsteady. 

Speech. — Modifications  and  alterations  of  the  sounds  pro- 
duced in  the  larynx  during  and  after  their  production  result, 
under  the  influence  of  the  sensorium,  in  articulate  speech.  These 
modifications  are  made  chiefly  by  the  tongue,  teeth,  and  lips. 

The  speech  sounds  are  divided  into  vowels  and  consonants. 
The  distinction  is  that  the  vowel  sounds  are  generated  in  the 


326  THE    SENSES 

larynx,  while  the  consonant  sounds  are  produced  by  alterations 
in  the  current  of  air  above  the  larynx,  and  cannot  be  pronounced 
except  constantly  with  a  vowel.  The  current  is  modified  mainly 
by  the  tongue  and  teeth  in  the  formation  of  linguals  and  dentals, 
by  the  cavity  of  the  nose  in  case  of  nasals,  and  by  changes  in  the 
shape  and  size  of  the  oral  cavity  in  the  production  of  other  sounds. 
Nervous  Supply  of  the  Larynx. — The  superior  laryngeal 
branch  of  the  teeth  is  the  sensory  nerve,  which  guards  the  glottis 
to  prevent  the  entrance  of  foreign  bodies.  Impressions  made 
on  the  filaments  of  this  nerve  are  reflected  through  the  medulla 
and  inferior  laryngeal  branch  of  the  tenth  to  the  muscles  which 
close  the  glottis.  The  inferior  laryngeal  also  innervates  the 
muscles  that  vary  the  tension  of  the  cords,  and  the  superior  laryn- 
geal keeps  the  mind  informed  of  the  state  of  these  muscles  and 
of  the  necessity  for  forced  expiration  or  coughing. 


CHAPTER  XIII. 
REPRODUCTION. 

VERY  many  facts  in  our  knowledge  of  reproduction  depend 
on  observations  made  upon  lower  animals,  but  there  is  sufficient 
analogy  between  the  known  facts  connected  with  human  repro- 
duction and  development  and  those  of  the  same  stages  in  other 
groups  of  beings  to  enable  us  to  present,  as  at  least  approxi- 
mately accurate,  certain  broad  principles  regarding  the  process 
as  it  pertains  to  the  human  race. 

In  order  that  a  human  being  may  be  brought  into  existence  it 
is  necessary  that  there  be  a  union  of  the  male  element,  the  sper- 
matozoon, and  the  female  element,  the  ovum.  Both  these  sexual 
cells  are  developed  from  epithelium — the  spermatozoon  from 
that  of  the  seminiferous  tubules  of  the  male,  and  the  ovum  from 
the  germinal  layer  of  the  ovary. 

In  what  follows  reference  will  be  had  to  reproductive  proc- 
esses in  the  human  being. 

Spermatozoa. — Human  spermatozoa  (Fig.  92)  are  elongated 
bodies,  about  one  five-hundredth  of  an  inch  in  length,  and  con- 
sist of  three  parts,  head,  mid-portion  and  tail.  The  last-named 
part  is  about  four-fifths  the  length  of  the  entire  spermatozoon. 
The  head  is  egg-shaped  and  much  the  thickest  part  of  the  ele: 
ment.  A  slender  filament,  the  axial  fiber,  extends  throughout 
its  length  from  head  to  tail  and  projects  slightly  beyond  the  latter. 
Spermatozoa  are  possessed  of  wonderful  vitality.  They  live  for 
several  weeks  in  the  genital  passages  of  the  female.  In  the  male 
genital  passages  they  may  live  for  months  in  a  quiescent  state. 
The  nucleus  is  the  fertilizing  agent.  Spermatozoa  are  also 

327 


328 


REPRODUCTION 


remarkable  for  their  power  of  locomotion,  which  is  effected  by 
lashings  and  rotary  movements  of  the  tail. 

Ova.— The  ovum  (Fig.  93),  or  female  sexual  cell,  is  the  largest 
cell  to  be  found  in  the  human  body.  Its  diameter  is  about  T ^-5-  of 
an  inch.  Its  structure  is  that  of  a  typical  cell.  When  the  ovary 


d 


k 

m 


FIG.  92. — Spermatozoa. 

i,  human(X  600),  the  head  seen  from  the  side;  a,  on  edge;  k,  head;  m,  middle 
piece;/,  tail;  e,  terminal  filament;  3,  from  the  mouse;  4,  bothriocephalus  latus;  5, 
deer;  6,  mole;  7,  green  woodpecker;  8,  black  swan;  9,  from  a  cross  between  a  gold- 
finch (m)  and  a  canary  (f.) ;  10,  from  cobitis.  (Landois.) 

is  developing  a  part  of  its  covering  epithelium  dips  down  into  the 
substance  of  the  organ  and  becomes  walled  off  by  union  of  the 
surface  cells  above  it.  A  part  of  this  ball  of  epithelium  becomes 
the  ovum,  and  a  part  the  Graafian  follicle  for  that  ovum.  The 
youngest  ova  are  thus  found  nearest  the  surface  of  the  ovary. 
The  cell  has  an  enveloping  membrane,  the  mtelline  membrane,  a 


GRAAFIAN   FOLLICLES 


329 


protoplasm,  the  vitellus,  a  nucleus,  the  germinal  vesicle,  and  a 
nucleolus,  the  germinal  spot.  Outside  the  ovum,  but  not  strictly 
a  part  of  it,  is  the  zona  pellucida,  a  transparent  envelope,  and 
outside  the  zona  pellucida  a  collection  of  cells,  the  corona  radiata. 
The  perivitelline  space  is  between  the  ovum  proper  and  the  zona 
pellucida.  The  zona  presents  radial  striae,  which  may  facilitate 
the  entrance  of  the  spermatozoon. 

Ova  are  capable  of  being  impregnated  as  long  as  7-9  days 
after  their  discharge  from  the  ovary.  Their  formation  begins 
early  in  fetal  life.  The  ovum  pos- 
sesses no  power  of  independent 
motion.  It  is  passive  in  fecun- 
dation; it  is  sought  by  the  male 
element.  Its  vitellus,  or  yolk 
(protoplasm),  contains  nutritive 
non-living  material,  deutoplasm, 
whose  function  is  to  furnish  food 
substance  to  the  impregnated 
ovum  until  the  fetal  circulation 
is  established.  Deutoplasm  in 
the  human  ovum  is  scarcely  to 
be  distinguished  from  the  living 
protoplasm,  though  in  the  ova 
of  birds,  e.  g.,  it  is  clearly 
marked  off,  and  constitutes  the 

main  bulk  of  the  mature  egg,  since  the  developing  embryo  re- 
ceives no  blood  from  the  mother. 

Graafian  Follicles. — The  Graafian  follicles  are  directly  con- 
cerned in  the  development  and  maturation  of  ova.  These  are 
small  vesicles  in  the  cortical  ovarian  substance  surrounded  by  a 
capsule  of  thickened  ovarian  stroma,  the  tunica  vasculosa.  In- 
side the  tunica  vasculosa,  lining  the  spherical  cavity  of  the  ves- 
icle, are  several  layers  of  epithelial  cells  making  up  the  membrana 
granulosa.  The  cavity  is  filled  with  an  albuminous  liquid,  the 


V 

FIG.  93. — Ovum.     (From  Yeo 
after  Robin.) 

a,  zona  pellucida  and  yitelline  mem- 
brane; b,  yolk;  c,  germinal  vesicle  or 
nucleus;  d,  germinal  spot  or  nucleolus; 
e,  interval  left  by  the  retraction  of  the 
vitellus  from  the  zona  pellucida. 


330 


REPRODUCTION 


liquor  folliculi.  At  one  point  in  its  circumference  the  membrana 
granulosa  is  much  thickened,  and  in  this  thickened  portion 
is  imbedded  the  ovum.  The  epithelial  cells  of  the  membrana 


FIG.  94. — Section  of  the  ovary  of  a  cat,  showing  the  origin  and  development 
of  Graafian  follicles.     (From  Yeo  after  Cadiat.} 

a,  germ  epithelium;  b,  Graafian  follicle  partly  developed;  c,  earliest  form  of 
Graafian  follicle;  d,  well-developed  Graafian  follicle;  e,  ovum;/,  vitelline  membrane; 
g,  veins;  h,  i,  small  vessels  cut  across. 

completely  surround  the  ovum,  constituting  the  discus  proligems. 
The  cells  of  the  discus  next  the  ovum  have  their  long  axes  at 


CORPUS    LUTEUM  331 

right  angles  to  the  circumference  of  the  egg,  and  this  layer  is  the 
corona  radiata  already  mentioned.  The  zona  pellucida  is  just 
underneath  the  corona. 

Usually  a  Graafian  follicle  contains  only  one  ovum.  The 
follicles  and  their  contained  ova  begin  to  be  formed  early  in 
fetal  life.  Probably  none  are  newly  formed  after  the  child  is  two 
years  old,  but  they  are  undeveloped  before  puberty.  It  is  esti- 
mated that  some  72,000  follicles  and  ova  exist  in  the  two  ovaries 
of  the  average  woman;  but  of  these  not  more  than  400  reach 
full  development,  the  others  undergoing  retrograde  changes  and 
disappearing. 

Up  to  puberty  the  follicles  and  ova  are  small,  but  at  that  time 
some  of  them  begin  to  enlarge,  and  at  more  or  less  regular  inter- 
vals one  of  these  follicles  bursts  and  allows  the  escape  of  its  con- 
tained ovum  into  the  fimbriated  extremity  of  the  Fallopian  tube 
— a  process  known  at  ovulation.  Previous  to  its  rupture  the 
Graafian  follicle  has  been  enlarging.  It  is  always  located  in  the 
cortical  part  of  the  ovary,  but  it  may  now  not  only  form  a  dis- 
tinct protrusion  above  the  surface  of  the  organ,  but  may  by  its 
size  encroach  upon  the  medullary  portion.  It  may  at  this  time 
have  a  diameter  of  half  an  inch.  Meantime  the  more  superficial 
part  of  the  tunica  vasculosa  has  been  undergoing  fatty  degenera- 
tion, has  lost  its  blood  supply  and  become  very  thin.  Here 
rupture  occurs,  and  the  mature  ovum,  ready  for  impregnation, 
escapes  upon  the  surface  of  the  ovary. 

Corpus  Luteum. — When  the  ovum  has  been  extruded  hem- 
orrhage occurs,  filling  the  empty  follicle  with  blood.  By  con- 
traction of  the  extra-vesicular  adjacent  tissue  the  walls  of  the 
Graafian  follicle  become  folded  into  the  cavity.  Soon  prolifera- 
tion of  the  cells  of  the  follicular  wall  takes  place  into  the  blood 
clot,  vascular  loops  are  formed,  and  the  tunica  vasculosa  itself 
becomes  greatly  hypertrophied.  The  clot  later  disappears  and 
the  mass  then  has  a  yellowish  color  and  is  known  as  the  corpus 
luteum. 


332 


REPRODUCTION 


Whether  or  not  the  ovum  that  escaped  from  the  follicle  which 
was  the  antecedent  of  any  given  corpus  luteum  was  impreg- 
nated, has  an  influence  upon  the  growth  of  that  corpus.  If  the 
ovum  failed  of  fecundation  the  corpus  luteum  will  reach  its  high- 
est development  in  about  fifteen  days,  and  will  then  assume 
the  character  of  cicatricial  tissue  and  be  absorbed  in  a  few  weeks. 
If  the  ovum  was  fecundated,  the  corpus  luteum  will  increase  in 
size  for  some  three  months,  until  it  may  be  half  the  size  of  the 
ovary.  At  labor  it  has  been  reduced  to  a  white  cicatrix,  which 
probably  persists  as  a  small  nodule  throughout  life.  The  differ- 
ences between  the  corpora  lutea  of  menstruation  and  pregnancy 
are  shown  by  the  following  table  from  Dalton: 


Corpus 
Luteum  of  Menstruation. 


Corpus 
Luteum  of  Pregnancy. 


At  the  end  of 
three  weeks. 
One  month. 


Two  months. 


Four  months. 


Six  months. 


Nine  months. 


Three-quarters  of  an  inch  in 
reddish;  convoluted  wall  pale. 


diameter;    central  clot 


Smaller;  convoluted  wall 
bright  yellow;  clot  still 
reddish. 

Reduced  to  the  condition 
of  an  insignificant  cicatrix. 


Absent  or  unnoticeable. 


Absent. 


Absent. 


Larger;  convoluted  wall 
bright  yellow;  clot  still 
reddish. 

Seven-eights  of  an  inch 
in  diameter;  convoluted;  wall 
bright  yellow;  clot  perfectly 
decolorized. 

Seven-eights  of  an  inch  in 
diameter;  clot  pale  and  fibrin- 
ous;  convoluted  wall  dull 
yellow. 

Still  as  large  as  at  the  end 
of  second  month;  clot  fibrin- 
ous;  convoluted  wall  paler. 

Half  an  inch  in  diameter; 
central  clot  converted  into  a 
radiating  cicatrix;  external 
wall  tolerably  thick  and  con- 
voluted, but  without  any 
bright  yellow  color. 


O  VITIATION  333 

Maturation. — But  previous  to  its  discharge  from  the  Graafian 
follicle,  the  ovum  undergoes  certain  changes— a  ripening  process 
— whereby  it  is  made  ready  to  receive  and  be  impregnated  by 
the  spermatozoon.  This  maturation  consists  in  the  discharge 
from  the  cell  proper  of  a  part  of  its  nucleus  and  a  part  of  its  pro- 
toplasm. The  nucleus  moves  toward  the  periphery,  and  the 
perinuclear  membrane  is  lost.  As  the  nucleus  approaches  the 
surface  of  the  egg  it  undergoes  karyokinesis,  and  a  part  of  it, 
together  with  a  little  surrounding  protoplasm,  is  extruded  and 


FIG.  95. — The  fertilized  ovum,  or  blastophere.     (Kirkes.) 

finds  itself  in  the  perivitelline  space.  This  is  the  first  polar  body. 
A  second  polar  body  is  likewise  later  discharged  by  karyokinetic 
division.  (See  Fig.  95.) 

The  object  of  this  extrusion  and  the  final  fate  of  the  polar 
bodies  are  matters  of  speculation.  That  portion  of  the  nucleus 
which  remains  after  the  polar  bodies  have  been  thrown  off  finds 
its  way  back  to  the  center  of  the  ovum.  It  soon  develops  a  cover- 
ing membrane,  and  is  now  the  female  pronudeus,  ready  for  union 
with  the  male  pronucleus.  It  is  about  the  time  of  the  completion 
of  this  process  that  the  follicle  ruptures  and  the  discharge  of  the 
ovum — ovulation — occurs. 

Ovulation. — It  is  supposed  that  from  puberty  to  the  meno- 
pause one  (or  more?)  ovum  is  discharged  at  tolerably  regular 


334  REPRODUCTION 

intervals  of  about  four  weeks.  It  should,  and  usually  does,  enter 
the  outer  end  of  the  Fallopian  tube,  to  be  conveyed  toward  the 
uterus.  Obviously  only  a  few,  and  sometimes  none,  are  ever 
impregnated.  Should  the  ovum  fail  to  reach  the  uterus  and 
become  fecundated,  ectopic  gestation  will  be  the  result. 

The  patent  nmbriated  extremity  of  the  tube  may  grasp  the 
ovary  at  the  time  of  rupture  of  the  Graafian  follicle,  but  this  is 
not  probable. "  One  of  the  tubal  fimbriae  is  attached  to  the  outer 
extremity  of  the  ovary  and  has  on  its  surface  a  small  linear  de- 
pression lined  by  ciliated  epithelium  and  leading  to  the  tube. 
The  ovum  very  likely  in  most  cases  drops  into  this  depression, 
and  the  influence  of  the  cilia  is  to  carry  it  toward  the  tube. 

Menstruation. — Usually  between  the  fourteenth  and  seven- 
teenth years  of  female  life  menstruation  begins.  It  is  a  discharge 
of  blood,  epithelium  and  other  parts  of  the  mucous  membrane  of 
the  uterine  cavity,  together  with  mucus  from  the  glands  of  the 
uterus  and  vagina.  About  the  beginning  of  menstrual  life  there 
are  marked  changes  in  bodily  development,  Graafian  follicles 
enlarge  and  begin  to  approach  the  surface,  ovulation  is  begun, 
and  the  female  is  capable  of  being  impregnated. 

In  most  cases  menstruation  occurs  at  regular  intervals  of 
twenty-eight  days.  The  function  is  suspended  during  pregnancy 
and  usually  during  lactation.  When  it  is  first  established  it  is 
frequently  irregular  in  its  occurrence  for  several  months;  a  like 
irregularity  usually  accompanies  the  cessation  of  the  function 
between  the  fortieth  and  fiftieth  years — when  the  menopause,  or 
climacteric,  is  established.  The  normal  female  may  be  impreg- 
nated during  menstrual  life,  but  not  before  or  after. 

The  average  length  of  time  for  which  the  menstrual  flow  con- 
tinues is  four  days.  There  are  many  exceptions  in  both  direc- 
tions for  different  women,  but  the  time  for  any  one  woman  prob- 
ably varies  little  under  normal  conditions.  The  discharge  for 
each  period  averages  some  five  ounces.  It  does  not  usually 
coagulate,  on  account  of  the  presence  of  alkaline  mucus.  For 


MENSTRUATION  335 

five  or  six  days  preceding  the  flow,  the  uterine  mucous  mem- 
brane gradually  thickens,  the  glands  become  longer  and  more 
tortuous,  the  connective  tissue  cells  multiply  and  the  blood-ves- 
sels are  greatly  increased  in  size.  This  is  apparently  a  prep- 
aration for  the  reception  of  the  impregnated  ovum.  A  short 
time  before  the  flow  begins  there  is  hemorrhage  into  the  subepi- 
thelial  tissue,  possibly  by  diapedesis,  possibly  by  rupture.  In  a 
day  or  so  the  superjacent  mucous  membrane  becomes  disinte- 
grated and  is  discharged  with  the  included  parts  of  the  glands. 
The  underlying  vessels,  being  thus  exposed,  rupture  and  the  san- 
guineous discharge  carries  away  the  debris. 

For  three  or  four  days  subsequent  to  the  cessation  of  the  flow 
the  uterine  mucosa  is  being  repaired.  The  deeper  layers,  in- 
cluding the  deeper  portions  of  the  glands,  were  not  cast  off,  and 
the  whole  is  reconstructed  from  the  intact  parts.  Following  the 
reconstructive  period  there  is  a  stage  of  quiescence  lasting  some 
two  weeks,  until  six  or  seven  days  prior  to  the  next  menstruation. 

At  the  beginning  of  each  menstrual  flow  there  is  general  con- 
gestion of  the  pelvic  viscera  and  mammary  glands,  accompanied 
usually  by  headache  and  a  sense  of  pelvic  oppression.  The 
congestion  and  discomfort  begin  to  disappear  when  the  flow  is 
established. 

Ovulation  probably  in  most  cases  takes  place  just  before  the 
menstrual  flow  begins,  but  neither  occurrence  is  dependent  upon 
the  other.  Ovulation  has  frequently  been  shown  to  take  place 
in  the  inter-menstrual  period,  but  the  congestion  of  the  repro- 
ductive organs  incident  to  menstruation  probably  hastens  the 
rupture  of  any  Graanan  follicle  which  at  that  time  happens  to 
be  near  the  completion  of  its  development. 

The  relations  between  ovulation,  menstruation  and  impregna- 
tion are  not  definitely  determined.  Pregnancy  lasts  for  ten 
lunar  months  and  dates  from  the  time  of  impregnation  (concep- 
tion), but  that  time  cannot  in  any  case  be  fixed  upon  with  pre- 
cision. The  vitality  of  the  ovum  is  thought  not  to  last  longer 


336  REPRODUCTION 

than  seven  days  unless  impregnated,  and  if  impregnation  is  to 
occur,  it  must  take  place  within  the  first  week  after  ovulation. 
Since,  therefore,  ovulation  and  menstruation  usually  occur  to- 
gether, and  since  impregnation  probably  occurs  about  the  be- 
ginning of  menstruation,  we  reckon  from  the  first  day  of  the  last 
menstruation  280  days  forward  to  determine  the  probable  time 
of  labor.  This  is  equivalent  to  adding  nine  calendar  months 
and  seven  days  to  the  first  day  of  the  last  menstrual  period.  It 
is  evident  that  this  calculation  at  best  gives  only  the  approxi- 
mate time. 

While  fertilization  probably  occurs  at  the  time  mentioned, 
the  spermatozoon  effecting  fecundation  may  have  been  in  the 
female  genital  tract  for  weeks.  Its  vitality  here  is  so  prolonged 
that  the  time  of  its  deposit  with  reference  to  menstruation  very 
probably  has  little  to  do  with  whether  or  not  conception  shall 
occur. 

Impregnation. — The  term  impregnation,  or  fertilization,  or 
fecundation,  is  used  to  signify  that  union  of  the  male  and  female 
sexual  cells  which  makes  possible  the  development  of  a  new 
human  being.  Normally  impregnation  takes  place  in  the  Fal- 
lopian tube,  and  almost  always  in  the  outer  third.  The  male 
element,  the  spermatozoon,  seeks  and  penetrates  the  female  ele- 
ment, the  ovum.  It  is  the  blending  of  the  nuclei  (pronuclei) 
which  is  essential.  Spermatozoa  in  large  numbers  swarm  around 
the  ovum  and  several  at  least  enter  the  perivitelline  space.  Only 
one,  however,  is  destined  usually  to  enter  the  ovum.  At  is  ap- 
proaches the  vitelline  membrane,  head  first,  the  protoplasm  of  the 
ovum  swells  up  into  a  prominence  to  meet  it.  The  fertilizing 
spermatozon  makes  its  way  through  the  vitelline  membrane, 
losing  its  tail  in  the  passage,  and  becomes  the  male  pronucleus. 
The  female  pronucleus  now  advances  from  its  central  position  to 
meet  the  male  element,  and  they  coalesce  to  become  the  segmen- 
tation nucleus.  Impregnation  has  now  taken  place.  The  seg- 
mentation nucleus  represents  a  new  being.  It  contains  anatom- 


SEGMENTATION  337 

ical  elements  from  both  parents,  and  it  is  not  surprising  that 
the  child  should  resemble  both,  anatomically  and  otherwise. 

The  term  "ovum"  has  so  far  been  used  to  signify  the  unim- 
pregnated  sexual  cell  discharged  from  the  female  ovary.  It  is 
also  used  to  signify  the  fertilized  cell,  and  is  in  fact  often  ap- 
plied without  much  precision  to  the  product  of  conception  at 
almost  any  stage  of  its  intrauterine  development. 

The  fertilized  ovum  is  carried  through  the  tube  to  the  uterus, 
arriving  there  some  seven  days  after  its  fecundation.  In  its  pas- 
sage it  becomes  covered  with  a  coating  of  albuminous  material. 
This  layer  is  probably  impervious  to  spermatozoa — which  fact 
may  account  for  the  practical  universality  of  fecundation  in  the 
outer  part  of  the  tube,  if  at  all.  The  coating  corresponds  to  the 
white  of  an  egg,  in  that  it  penetrates  the  perivitelline  membrane 
and  furnishes  nutritive  material  to  the  vitellus.  On  reaching  the 
uterus  the  ovum  becomes  attached  to  and  covered  by  the  thick- 
ened mucous  membrane  of  that  organ  in  a  way  to  be  noted  pres- 
ently. Here  it  remains  until  expelled  during  parturition. 

Segmentation. — As  soon  as  union  of  male  and  female  pro- 
nuclei  has  taken  place,  cleavage  of  the  ovum  begins.  The  nu- 
cleus (segmentation  nucleus)  and  protoplasm  divide  by  karyo- 
kinesis  to  form  two  nearly  similar  cells.  These  two  divide  into 
four,  these  four  into  eight  and  so  on,  till  a  large  number  of  cells 
occupy  the  vitelline  space  and  are  all  surrounded  by  the  perivi- 
telline membrane.  As  division  proceeds,  cells  arrange  them- 
selves around  others,  so  that  the  former  occupy  the  circumference 
and  the  latter  the  center  of  the  vitelline  cavity.  Later,  while 
the  outer  cells  constitute  a  layer  covering  the  entire  inner  surface 
of  the  perivitelline  membrane,  the  inner  cells  group  to  form  a 
mass  which  is  in  contact  with  the  outer  layer  at  one  point  only— 
like  a  ball  lying  in  a  relatively  large  hollow  sphere.  The  space 
thus  left  between  the  two  kinds  of  cells  is  called  the  segmentation 
cavity.  Soon  the  surrounding  cells  become  attenuated  (Rauber's 
cells)  and  disappear.  Their  place,  as  a  surrounding  envelope, 


338 


REPRODUCTION 


.is  taken  by  some  of  the  cells  of  the  inner  layer.  This  second 
surrounding  layer  is  the  epiblast,  or  ectoderm;  the  surrounded 
mass  is  the  hypoblast,  or  entoderm. 


ent. 


FIG.  96. — Sections  of  the  ovum  of  a  rabbit,  showing  the  formation  of  the 
blastodermic  vesicle.     (From  Yeo  after  E.  Van  Beneden.) 

a,  b,  c,  d,  are  ova  in  successive  stages  of  development;  Z,  p,  zona  pellucida;  ect, 
ectomeres,  or  outer  cells;  ent,  entomeres,  or  inner  cells. 

Before  long  the  entoderm  spreads  out  over  a  larger  area,  and 
from  it  and  from  the  ectoderm  is  developed  a  layer  of  cells,  the 


DERIVATIVES    OF    THE    GERM   LAYERS 


339 


mesoblast,  or  mesoderm,  which  occupies  a  position  between  the 
other  two  layers.  The  three-layered  germ  is  now  the  blasto- 
dermic  vesicle,  or  the  gastrula,  and  its  cavity  is  the  archenteron,  or 
celenteron.  From  these  three  germ  layers  are  developed  all  the 
parts  of  the  body  by  the  formation  of  folds,  ridges,  constrictions, 
etc.,  and  by  various  metamorphoses  which  have  as  their  end  the 
adaptation  of  structure  to  function. 

Derivatives  of  the  Germ  Layers.— According  to  Heisler 
these  are: 

From  the  ectoderm:  (i)  The  epidermis  and  its  appendages, 
including  the  nails,  the  hair,  the  epithelium  of  the  sebaceous 
and  sweat  glands  and  the  epithelium  of  the  mammary  gland. 
(2)  The  infoldings  of  the  epidermis,  including  the  epithelium  of 
the  mouth  and  salivary  glands,  of 
the  nasal  tract  and  its  communi- 
cating cavities,  of  the  external  audi- 
tory canal,  of  the  anus  and  anterior 
urethra,  of  the  conjunctiva  and  an- 
terior part  of  the  cornea,  the  ante- 
rior lobe  of  the  pituitary  body,  the 
crystalline  lens  and  the  enamel  of 
the  teeth.  (3)  The  spinal  cord  and 
brain  with  its  outgrowths,  including 
the  optic  nerve,  the  retina  and  the 
posterior  lobe  of  the  pituitary  body. 
(4)  The  epithelium  of  the  internal 
ear. 

From  the  entoderm:  The  epithe- 
lium of  the  respiratory  tract,  of  the 
digestive  tract  (from  the  back  part  of  the  pharynx  to  the  anus, 
including  its  associated  glands,  the  liver  and  pancreas),  of  the 
middle  ear  and  Eustachian  tube,  of  the  thymus  and  thyroid 
bodies,  of  the  bladder  and  first  part  of  the  male  urethra  and  of 
the  entire  female  urethra. 


FIG.  97. — Impregnated  egg. 

With  commencement  of  forma- 
tion of  embryo;  showing  the  area 
germinaiiva  or  embryonic  spot,  the 
area  pellucida,  and  the  primitive 
groove  and  streak.  (Kirkes  after 
Dalton.) 


340  REPRODUCTION 

From  the  mesoderm:  (i)  Connective  tissue  in  all  its  forms,  such 
as  bone,  dentine,  cartilage,  lymph,  blood,  fibrous  and  areo- 
lar  tissue;  (2)  muscular  tissue;  (3)  all  endothelial  cells;  (4) 
the  spleen,  kidney  and  ureter,  testicle  and  its  excretory  ducts,  uterus, 
Fallopian  tube,  ovary  and  vagina. 

The  Embryonal  Area.— Soon  after  the  germ  reaches  the 
uterus  (probably)  there  appears  on  its  surface  on  oval  whitish 
spot,  the  embryonal  area.  The  impregnated  ovum  is  still  in  the 
shape  of  a  vesicle.  It  is  from  the  embryonal  area  alone  that  the 
body  is  developed.  The  other  parts  are  accessory.  Longi- 
tudinal division  of  this  area  is  supposed  to  give  rise  to  twins  of 
the  same  sex  and  of  almost  identical  structure.  Running  in  the 
long  diameter  of  the  embryonal  area  is  a  marking,  the  primitive 
streak,  in  which  is  a  longitudinal  depression,  the  primitive 
groove.  (Fig.  97.)  These  surface  markings  are  caused  by 
thickening  of  the  ectoderm.  (Fig.  98.) 

Development  of  Mesoderm. — It  is  about  this  time  that  the 
mesoderm  makes  its  appearance.  It  begins  under  the  primitive 
groove  and  extends  in  all  directions.  It  originates  from  both 
ectoderm  and  entoderm,  and  lies  between  them.  In  the  median 
line  the  three  layers  are  closely  united  to  each  other.  (Fig.  98.) 
At  first  the  mesoderm  does  not  completely  embrace  the  germ, 
but  is  deficient  opposite  the  embryonal  area. 

Fig.  94  shows  that  the  cells  of  the  mesoderm  make  up  a  thick- 
ened mass  near  the  median  line,  but  farther  away  they  constitute 
two  distinct  lamellae.  The  mass  near  the  median  line  is  the 
vertebral  or  axial  plate.  The  outer  of  the  lateral  lamellae  is  the 
somatic  mesoderm;  the  inner  is  the  splanchnic  mesoderm.  The 
ectoderm  and  somatic  mesoderm  unite  to  form  the  somatopleure; 
the  entoderm  and  splanchnic  mesoderm  unite  to  form  the  splan- 
chnopleure.  The  interval  left  between  the  somatopleure  and 
splanchnopleure  is  the  celom,  or  body  cavity.  (Fig.  98.)  The 
great  serous  cavities  of  the  body  are  developed  from  it. 

Beginning  Differentiation. — It  thus  appears  that  the  embryo 


BEGINNING   DIFFERENTIATION 


341 


is  beginning  to  develop  from  the  simple  vesicle  into  specialized 
parts. 

We  shall  notice  briefly  the  development  of  the  body  proper,  and 


the  extra-embryonic  accessory  structures,  the  umbilical  vesicle, 
amnion,  allantois  and  placenta.  As  regards  the  embryonic  body, 
some  of  the  most  prominent  occurrences  connected  with  its 


342 


REPRODUCTION 


development  consist  in  the  formation  of  the  neural  canal,  chorda 
dorsalis,  or  notochord,  and  mesoblastic  somites. 

Neural  Canal.— About  the  fourteenth  day,  along  underneath 


the  primitive  groove,  the  cells  of  the  ectoderm  become  thickened 
to  form  the  medullary  plate.  The  edges  of  this  longitudinal 
plate  soon  begin  to  curl  up,  and  thus  form  the  medullary  furrow, 
or  groove.  (Fig.  99.)  The  margins  of  the  adjacent  ectoderm 


CHORDA   DORSALIS 


343 


are  carried  up  with  the  curling  edges,  and  constitute  the  medullary 
folds.  Later  the  edges  of  the  medullary  plate  meet  each  other, 
and  join  to  form  a  closed  canal,  the  neural  or  medullary  canal. 
The  edges  of  the  medullary  folds  unite  above,  so  that  the  neural 
canal  comes  to  lie  underneath  the  surface  ectoderm.  (Fig.  100.) 
The  neural  canal  is  the  forerunner  of  the  whole  nervous  system. 
Chorda  Dorsalis. — The  method  of  formation  of  the  chorda 
dorsalis,  or  notochord,  is  very  similar  to  that  of  the  neural  canal. 


FIG.    100. — Transverse   section   through   dorsal   region   of    embryo   chick 

(45  hours) . 

One-half  of  the  section  is  represented;  if  completed  it  would  extend  as  far  to  the 
left  as  to  the  right  of  the  line  of  the  medullary  canal  (Me).  A,  epiblast:  t7,  hypoblast, 
consisting  of  a  single  layer  of  flattened  cells;  Me,  medullary  canal;  Pv,  protovertebra ; 
Wd,  Wolffian  duct;  So,  somatopleure;  Sp,  splanchnopleure;  pp,  pleuroperitoneal 
cavity;  ch,  notochord;  ao,  dorsal  aorta,  containing  blood-cells;  v,  blood-vessels  of 
the  yolk-sac.  (Kirkes  after  Foster  and  Balfour.) 


It  is  a  solid,  instead  of  a  cylindrical,  longitudinal  collection  of 
cells,  extending  along  the  dorsal  aspect  of  the  celom.  It  is 
developed  from  the  entoderm.  A  thickening  of  the  cells  of  this 
layer  constitutes  the  chordal  plate.  Its  edges  curl  up  in  a  direc- 
tion opposite  to  those  of  the  medullary  plate,  and  carry  with  them 
chordal  folds  of  the  entoderm.  When  the  curling  edges  have 
joined  to  form  a  solid  cylinder  of  cells,  the  chordal  folds  unite 
over  the  ventral  surface  of  the  cylinder.  Figures  99  and  100 


344  REPRODUCTION 

illustrate  these  facts.  The  notochord  is  in  the  line  of  the  future 
vertebral  bodies,  but  it  is  not  developed  into  any  adult  structure. 

Somites. — These  are  masses  of  cells  developed  from  the  axial 
plates  of  the  mesoderm,  lying  parallel  with  and  on  each  side  of 
the  notochord.  (Fig.  100.)  They  are  in  segments,  the  forma- 
tion of  which  begins  in  the  neck  and  proceeds  caudad  and  cepha- 
lad.  They  are  sometimes  called  the  protovertebrce.  They 
represent  the  primitive  vertebrae. 

The  body  begins  to  assume  shape  and  the  fetal  appendages  to 
be  developed  at  the  same  time.  The  latter  are  for  the  protection 
and  nutrition  of  the  embryo.  The  essential  parts  of  a  vertebrate 
are  a  vertebral  column  with  a  neural  canal  above  and  a  body  cavity 
below  it.  The  body  cavity  contains  the  alimentary  canal. 
The  somites  representing  the  vertebral  column  and  the  formation 
of  the  neural  canal  have  been  noticed. 

Body  Cavity. — At  first  the  embryo,  as  represented  by  the 
embryonal  area,  is  on  a  level  with  the  remaining  surface  of  the 
blastoderm.  Soon,  however,  there  appears,  marking  the  head 
of  the  embryo  and  with  its  concavity  backward,  a  crescentic 
folding  in  of  the  blastodermic  wall.  It  is  evident  on  the  surface 
as  a  simple  furrow.  This  tucking-in  finally  surrounds  the  whole 
embryonal  area,  and  the  surface  fissure,  now  oval,  becomes 
deeper  and  deeper,  until  those  portions  of  the  wall  which  are 
being  tucked  under  the  embryo  approach  each  other  on  its 
ventral  aspect  and  divide  the  yolk  into  two  communicating  cav- 
ities. (See  Figs.  102  and  103.) 

The  layers  of  the  blastoderm  thus  folded  underneath  the  em- 
bryo are  the  visceral  plates.  They  form  the  boundaries  of  a 
cavity  which  still  communicates  in  front,  at  the  site  of  the  future 
umbilicus,  with  the  yolk-sac.  This  narrow  canal  is  the  vitelline 
duct,  and  the  two  cavities  communicating  through  the  vitelline 
duct  are  the  future  alimentary  canal  and  the  yolk-sac,  or  umbil- 
ical vesicle.  It  is  to  be  noticed  that  the  visceral  plates  embrace 
both  somatopleure  and  splanchnopleure,  and  that  it  is  the 


BODY   CAVITY  345 

ectodermic  layers  of  the  splanchnopleure  which  finally  join  to 
form  the  gut  tract,  and  the  somatopleure  which  forms  the  ventral 
and  lateral  walls  of  the  body  cavity.  The  gut  tract  has  the 


FIG.  101. — Diagrammatic  section  showing  the  relation  in  a  mammal  between 
the  primitive  alimentary  canal  and  the  membrane  of  the  ovum. 

The  stage  represented  in  this  diagram  corresponds  to  that  of  the  fifteenth  or  seven- 
teenth day  in  the  human  embryo,  previous  to  the  expansion  of  the  allantois;  c,  the 
villous  chorion;  a,  the  amnion;  a',  the  place  of  convergence  of  the  amnion  and  re- 
flexion of  the  false  amnion;  a"  a",  outer  or  corneous  layer;  e,  the  head  and  trunk  of 
the  embryo,  comprising  the  primitive  vertebrae  and  cerebro-spinal  axis;  i,  i,  the 
simple  alimentary  canal  in  its  upper  and  lower  portions.  Immediately  beneath  the 
right  hand  i  is  seen  the  fetal  heart,  lying  in  the  anterior  part  of  the  pleuroperitoneal 
cavity;  v,  the  yolk-sac  or  umbilical  vesicle;  vi,  the  vitello-intestinal  opening;  M,  the 
allantois  connected  by  a  pedicle  with  the  hinder  portion  of  the  alimentary  canal. 
(Kirkes  after  Quain,) 


shape  of  a  straight  tube  occupying  the  long  axis  of  the  embryo 
and  opening  into  the  umbilical  vesicle. 


346  REPRODUCTION 

Fetal  Membranes. 

Umbilical  Vesicle. — The  umbilical  vesicle  represents  that 
part  of  the  vitellus  which  has  not  been  constricted  off  to  form 
the  gut  tract.  (Figs.  101,  102,  103.)  It  furnishes  nutriment  to 
the  embryo  for  a  short  time  and  is  then  largely  cut  off  from 
the  body.  It  gradually  shrivels  (Figs.  107,  108),  and  with  that 
part  of  the  duct  external  to  the  abdomen  is  cast  off  either  before 


FIGS.  1 02  AND  103. 

a,  chorion  with  villi.  The  yilli  are  shown  to  be  best  developed  in  the  part  of  the 
chorion  to  which  the  allantois  is  extending;  this  portion  ultimately  becomes  the 
placenta;  b,  space  between  the  true  and  false  amnion;  c,  amniotic  cavity;  d,  situation 
of  the  intestine,  showing  its  connection  with  the  umbilical  vesicle;  e,  umbilical 
vesicle;/,  situation  of  heart  and  vessels;  g,  allantois.  (Kirkes.) 


or  at  parturition.  .Vessels  develop  in  its  walls  and  absorb  the 
nourishment  in  it  to  be  conveyed  to  the  embryo.  But  in  the 
human  being  more  satisfactory  arrangements  for  nutrition  are 
soon  made  and  its  function  ceases. 

Amnion. — When  the  embryo  has  become  depressed,  as  it 
were,  into  the  substance  of  the  blastoderm,  and  while  the  body 
cavity  is  being  formed,  the  layers  of  the  somatopleure  grow  up 
over  the  embryo  to  meet  and  blend  dorsally.  (Figs.  107,  108.) 
The  two  layers  of  which  the  somatopleure  is  composed  separate , 


THE  ALLANTOIS 


347 


the  outer  forming  the  false  amnion  and  the  inner  the  true  amnion. 
The  false  amnion  now  coalesces  with  the  original  vitelline  mem- 
brane to  constitute  the  false  chorion.  Evidently  there  is  thus 
formed  a  closed  cavity,  the  amniotic  cavity,  between  the  true  am- 
nion and  the  body  of  the  embryo. 

At  first  the  amnion  and  the  embryo  are  in  close  contact,  but 
soon  the  cavity  begins  to  be  distended  with  the  fluid,  the  liquor 


FIG.  104. — Diagram  of 
fecundated  egg. 

a,  umbilical  vesicle;  b, 
amniotic  cavity;  c,  allan- 
tois.  (Kirkes  after  Dai- 
ton.) 


FIG.  105. — Fecundated  egg  with  allantois 
nearly  complete. 

a,  inner  layer  of  amniotic  fold;  b,  outer  layer  of 
ditto;  c,  point  where  the  amniotic  folds  come  in  con- 
tact. The  allantois  is  seen  penetrating  between  the 
outer  and  inner  layers  of  the  amniotic  folds.  This 
figure,  which  represents  only  the  amniotic  folds  and 
the  parts  within  them,  should  be  compared  with  Figs. 
99,  100,  in  which  will  be  found  the  structures  external 
to  these  folds.  (Kirkes  after  Dalton.) 


amnii,  which  increases  until  it  reaches  a  considerable  quantity. 
It  affords  mechanical  protection  to  the  fetus  during  intrauterine 
life,  and  at  labor  serves  to  evenly  dilate  the  cervix.  When  this 
has  been  accomplished  is  the  usual  time  at  which  the  sac  ruptures 
and  the  liquor  amnii  escapes.  It  also  supplies  the  fetal  tissues 
with  water,  parts  of  it  being  swallowed  from  time  to  time. 

The  cavity  between  the  false  amnion  and  the  true  amnion  is 
continuous,  with  the  body  cavity  at  the  umbilicus. 

Allantois. — The  allantois  grows  out  from  the  back  part  of  the 
intestinal  canal  into  the  celom  or  the  body  cavity.  (Figs.  104, 
105).  It  is  of  splanchnopleuric  origin.  It  soon  becomes  a  mem- 


348 


REPRODUCTION 


branous  sac,  the  walls  of  which  are  very  vascular.  It  fills  the 
space  between  the  two  amniotic  folds  and  joins  the  false  amnion. 
Its  vessels  thus  reach  the  chorion,  which  is  already  establishing 
vascular  connections  with  the  mother.  Finally  they  are  distrib- 


FIG.  106. — This  and  the  two  following  wood-cuts  are  diagrammatic  views 
of  sections,  through  the  developing  ovum,  showing  the  formation  of  the 
membranes  of  the  chick.  (Yeo,  after  Foster  and  Balfour.} 

A,  B,  C,  D,  E,  and  F,  are  vertical  sections  in  the  long  axis  of  the  embryo  at  differ- 
ent periods,  showing  the  stages  of  development  of  the  amnion  and  of  the  yolk-sac; 
I,  II,  III,  and  IV,  are  transverse  sections  at  about  the  same  stages  of  development; 
i,  ii,  and  iii,  give  only  the  posterior  part  of  the  longitudinal  section  to  show  three 
stages  in  the  formation  of  the  allantois;  e,  embryo;  y,  yolk;  pp,  pleuroperitoneal 
fissure;  vt,  vitelline  membrane;  a/,  amniotic  fold;  a/,  allantois. 


uted  only  to  a  certain  part  (placenta)  of  the  chorion;  and  as  the 
allantoic  vessels  anastomose  more  and  more  freely  with  those 
of  the  chorion,  the  umbilical  vesicle  shrivels,  as  it  is  no  longer 
needed.  The  vessels  of  the  allantois  are  the  two  allantoic 


THE  ALLANTOIS 


349 


arteries  and  the  same  number  of  allantoic  veins.     The  allantois 
also  receives  the  fetal  urine. 
As  the  true  placental  circulation  is  established  and  the  visceral 


PP- 


FIG.  107 


< 

e,  embryo;  a,  amnion;  a',  alimentary  canal;  vt,  vitelline  membrane;  a/,  amniotic 
fold;  ac,  amniotic  cavity;  y,  yolk;  al,  allantois. 

plates  close  the  abdominal  cavity,  the  allantois  is  constricted 
at  the  umbilicus  so  as  to  be  divided  into  two  parts.  That 
outside  the  body  shrivels  and  is  cut  away  with  the  umbilical 


350 


REPRODUCTION 


cord  at  birth,  while  that  inside  the  body  becomes  the  first  part 
of  the  male  and  the  whole  of  the  female  urethra,  the  bladder 
and  the  urachus. 

Chorion. — The  chorion  is  the  outer  surrounding  membrane 
of  the  embryo  after  the  appearance  of  the  amnion.     It  consists  of 


FIG.  108. — Diagrammatic  sections  of  an  embryo. 

Showing  the  destiny  of  the  yolk-sac,  ys.     vt,  vitelline  membrane;  pp,  pleuroperit- 
oneal  cavity;  ac,  cavity  of  the  amnion;  a,  amnion;  a',  alimentary  canal;  ys,  yolk-sac. 


three  layers.  From  without  inward  these  are  the  original 
vitelline  membrane,  the  false  amnion  and  the  allantois.  The 
allantois  has  been  seen  to  extend  around  between  the  two  amni- 
otic  folds  and  to  blend  with  the  outer.  From  its  formation  from 
these  several  membranes,  the  chorion  evidently  consists  of  the 
outer  ectodermic,  inner  entodermic  and  intervening  mesodermic 
strata. 


THE   DECIDUA  351 

By  the  time  the  impregnated  ovum  reaches  the  uterus,  the 
chorion  (false  at  this  time)  has  numerous  spike-like  projections 
— villi — over  its  whole  surface.  (Fig.  101.)  These  are  at  first 
non-vascular,  but  soon  become  vascular  by  the  projection  into 
them  of  capillaries  from  the  vessels  of  the  allantois.  These 
capillaries  probably  absorb  nutrient  matter  secreted  by  the 
uterine  glands.  But  at  the  beginning  of  the  third  month  the 
villi  become  much  more  highly  developed  over  a  certain  part  of 
the  surface  of  the  chorion  than  at  other  points,  and  a  more 
intimate  relation  is  established  between  their  vessels  and  those 
of  the  mother;  here  the  placenta  is  to  be  formed. 

The  Decidua. — The  decidua  of  pregnancy  consists  of  the 
hypertrophied  mucous  membrane  lining  the  cavity  of  the  uterus 
and  reflected  at  a  certain  point  entirely  over  the  developing  ovum. 
Before  the  ovum  reaches  the  uterus,  the  mucous  membrane  of 
the  latter  has  been  undergoing  changes,  such  are  mentioned 
under  Menstruation.  If  fecundation  has  not  taken  place, 
menstruation  occurs  and  the  mucosa  is  discharged  under  the 
name  of  the  decidua  menstrualis.  But  if  conception  has  oc- 
curred, menstruation  does  not  ensue  and  the  uterine  mucosa 
becomes  much  more  thick  and  spongy.  Whether  or  not  it 
shall  be  discharged  as  the  decidua  of  menstruation  or  be  retained 
to  form  the  decidua  of  pregnancy  is  probably  a  point  which  is 
decided  while  the  ovum  is  yet  in  the  tube. 

When  the  fecundated  ovum  reaches  the  uterus  it  becomes 
attached  to  the  mucous  membrane,  usually  a  little  to  one  side  of 
the  median  line  on  the  posterior  wall.  The  mucous  membrane 
extends  over  and  completely  envelops  it.  This  reflected  portion 
is  the  decidua  reflexa;  that  lining  the  whole  uterine  cavity  is  the 
decidua  vera,  while  that  part  of  the  decidua  vera  intervening 
between  the  ovum  and  the  uterine  wall  is  the  decidua  serotina 
and  becomes  the  maternal  part  of  the  placenta. 

Of  course  there  is  at  first  a  considerable  cavity  left  between  the 
reflex  and  the  vera,  but  as  the  embryo  increases  in  size  the 


352 


REPRODUCTION 


space  becomes  smaller  and  is  obliterated  by  the  end  of  the  fifth 
month.  After  this  time  both  vera  and  reflexa  undergo  retro- 
grade changes  due  to  pressure  and  become  closely  attached  to 


FIG.  109. — Diagrammatic  view  of  a  vertical  transverse  section  of  the  uterus 
at  the  seventh  or  eighth  week  of  pregnancy. 

c,  c,  c',  cavity  of  uterus,  which  becomes  the  cavity  of  the  decidua,  opening  at  c,  c, 
the  cornua,  into  the  Fallopian  tubes,  and  at  ff  into  the  cavity  of  the  cervix,  which  is 
closed  by  a  plug  of  mucus;  dv,  decidua  vera;  dr,  decidua  reflexa,  with  the  sparser  villi 
imbedded  in  its  substance;  ds,  decidua  serotina,  involving  the  more  developed 
chorionic  vili  of  the  commencing  placenta.  The  fetus  is  seen  lying  in  the  amniotic 
sac;  passing  up  from  the  umbilicus  is  seen  the  umbilical  cord  and  its  vessels  passing 
to  their  distribution  in  the  villi  of  the  chorion;  also  the  pedicle  of  the  yolk-sac,  which 
lies  in  the  cavity  between  the  amnion  and  chorion.  (Kirkes  after  Allen  Thomson.) 


the  chorion.    They  are  discharged  with  the  membranes  at  birth. 
Placenta.— The  placenta  is  the  organ  of  nutrition  for  the 


THE  PLACENTA  353 

fetus  after  about  the  end  of  the  third  month.  Through  it  the 
vessels  of  the  fetus  and  those  of  the  mother  are  brought  into  most 
intimate  relations. 

It  has  been  said  that  the  villi  of  the  chorion  in  one  locality 
become  very  highly  developed.  This  is  at  the  site  of  the  reflec- 
tion of  the  decidua  serotina  and  is  the  chorion  f rondo  sum.  The 
union  of  these,  with  certain  other  developments,  constitutes  the 
placenta. 

The  decidua  serotina  becomes  very  spongy.  It  is  filled  with 
sinuses,  into  which  the  enlarged  villi  of  the  chorion  frondosum 
project.  The  sinuses  are  filled  with  maternal  blood,  while  the 
capillaries  of  the  villi  contain  fetal  blood.  There  is  no  direct 
connection  between  the  vessels  of  mother  and  child,  but  the  thin 
lining  of  the  villi  and  sinuses  allows  free  interchange  of  materials 
by  osmosis. 

It  seems  that  the  interchange  is  under  the  influence  of  two 
sets  of  cells,  each  disposed  in  a  single  layer — one  belonging  to  the 
maternal  and  the  other  to  the  fetal  part  of  the  placenta.  These 
layers  of  cells  are  situated  on  either  side  of  the  membrane  of  the 
villus.  They  seem  to  take  out  of  the  maternal  blood  materials 
needed  for  the  nutrition  of  the  fetus,  and  out  of  the  fetal  blood 
materials  which  require  removal.  The  maternal  blood  per- 
forms both  alimentary  and  respiratory  functions  for  the  fetus. 

The  placenta  as  a  whole  is  discoid  in  shape.  Its  fetal  surface 
is  concave  and  covered  by  the  amnion.  The  mass  has  a  diame- 
ter of  4-5  in.,  and  a  thickness  of  half  an  inch.  The  villi  receive 
blood  from  the  allantoic  or  umbilical  arteries;  it  is  returned  by 
the  umbilical  vein. 

At  labor  uterine  contractions  detach  the  placenta  and  the 
decidua  and  expel  them  from  the  womb.  The  separation  takes 
place  in  the  deeper  part  of  the  maternal  placenta,  or  decidua 
serotina,  so  that  the  mass  discharged  represents  both  the  fetal 
and  maternal  portions.  The  vessels  entering  the  sinuses  do 
so  obliquely;  consequently  uterine  contractions '  at  birth  very 
23 


354  REPRODUCTION 

effectually  check  the  hemorrhage  which  separation  of  the  pla- 
centa occasions. 

Umbilical  Cord. — The  umbilical  cord  is  made  up  of  the 
vessels  which  convey  blood  between  the  placenta  and  fetus,  to- 
gether with  the  remnants  of  the  umbilical  vesicle  and  allantoic 
stalk,  all  of  which  are  held  together  by  the  jelly  of  Wharton,  a 
species  of  connective  tissue. 

The  outgrowing  allantois  has  developed  in  it  the  two  allantoic 
arteries  and  veins.  By  the  time  the  placenta  is  formed  the 
allantoic  stalk  has  become  much  elongated,  and  the  allantoic 
vessels  extend  into  the  fetal  placenta  (chorion  frondosum)  and 
become  now  the  umbilical  vessels.  The  two  veins  blend  to 
constitute  a  single  umbilical  vein,  but  the  arteries  remain  sep- 
arate. The  vein  enters  the  fetal  body  at  the  umbilicus,  passes 
to  the  under  surface  of  the  liver  and  divides  in  a  manner  to  be 
noted  presently.  After  birth  the  intra-abdominal  portion 
atrophies,  and  is  the  round  ligament  of  the  liver.  The  two 
umbilical  arteries  issue  at  the  umbilicus.  Their  intra-abdominal 
portions  are  the  fetal  hypogastric  arteries. 

The  average  length  of  the  umbilical  card  is  about  twenty-one 
inches.  It  appears  to  be  twisted  on  account  of  the  spiral  course 
of  its  relatively  long  arteries.  It  is  usually  attached  near  the 
center  of  the  fetal  surface  of  the  placenta. 

Condition  of  the  Fetal  Membranes  at  Birth. — The  mem- 
branes discharged  with  the  placenta  at  birth  are,  from  without 
inward,  the  decidua  vera,  decidua  reflexa,  chorion  and  amnion. 
The  amniotic  fluid,  in  which  the  fetus  floats,  reaches  its  maxi- 
mum amount  at  about  the  sixth  month.  It  is  sufficient  then  to 
force  the  amnion  closely  against  the  chorion,  covered  by  the 
decidua  reflexa;  these  last  named  (chorion  and  reflexa)  are  in 
turn  forced  everywhere  against  the  decidua  vera.  The  result 
is  that  all  four  become  practically  one  membrane,  though  the 
union  between  amnion  and  chorion  is  not  so  close  as  that  between 
the  other  layers.  These  membranes  constitute,  then,  a  sac 


VITELLINE    CIRCULATION  355 

filled  with  fluid.  The  sac  is  ruptured  in  labor,  and  the  child 
escapes  through  the  rent.  Afterward  the  decidua  vera  and 
placenta  are  detached,  and  escape  together  as  the  "  after  birth." 

Development  of  the  Circulation. — The  development  of  the 
circulation  may  be  considered  in  these  stages:  (i)  Vitelline 
circulation,  (2)  placental  circulation,  (3)  adult  circulation.  The 
heart  is  the  propelling  organ  in  all  these. 

i.  Vitelline  Circulation. — The  blood  and  vessels  make  their 
appearance  almost  as  early  as  the  primitive  groove.  Certain 
blastodermic  cells  are  transformed  into  both  red  and  white 
corpuscles.  They  are  larger  than  the  adult's  cells  and  both 
are  nucleated.  Blastodermic  cells  also  group  to  form  small 
tubes,  which  constitute  the  area  vasculosa.  At  the  same  time 
mesoblastic  cells  develop  two  tubes,  one  along  each  side  of  the 
body,  which  soon  unite  to  form  a  single  one,  representing  the 
heart.  It  becomes  enlarged  and  twisted  upon  itself,  and  pulsa- 
tions begin  in  it  at  a  very  early  date.  The  heart  is  in  the  median 
line  and  gives  off  two  arches  which  unite  below  to  form  the 
abdominal  aorta.  From  the  arches  pass  branches  to  the  area 
vasculosa,  which  now  form  a  nearly  circular  plexus  around  the 
embryo.  Two  of  these  branches,  larger  than  the  others,  enter 
the  umbilical  vesicle  and  become  the  omphalo-mesenteric  arteries; 
there  are  two  corresponding  veins.  This  circulation  through  the 
omphalo-mesenteric  vessels  and  the  area  vasculosa  does  not 
continue  long  in  the  human  being.  As  soon  as  the  allantois  is 
formed  and  the  placental  circulation  begins  to  be  set  up,  the 
omphalo-mesenteric  vessels  are  obliterated  and  the  place  of  the 
first  circulation  is  taken  by  the  second. 

Development  of  the  Heart. — The  tube  just  mentioned  as  rep- 
resenting the  heart  has  communicating  with  it  two  veins  at  its 
lower  extremity  and  two  arteries  at  its  upper.  Soon  the  tube 
becomes  twisted  upon  itself  so  that  the  upper  (arterial)  is  thrown 
in  front  of  the  lower  (venous).  The  loop  is  V-shaped  and  is 
the  outline  of  the  future  ventricles.  Afterward  a  constriction 


356  REPRODUCTION 

forms  the  auricle.  At  this  time- the  heart  consists  of  a  single 
ventricle  and  a  single  auricle.  Later  the  ventricular  and  auric- 
ular septa  are  formed.  The  latter  appears  after  the  former  and 
is  incomplete ;  the  opening  left  between  the  auricles  is  the  fora- 
men ovale. 

2.  Placental  Circulation. — As  the  allantois  is  developed  and 
the  vitelline  circulation  is  abolished,  the  hypogastric  arteries 
are  given  off  first  from  the  aorta,  but  later  (with  the  develop- 
ment of  the  vessels  of  the  lower  extremities)  they  are  pushed 
down,  as  it  were,  so  that  they  take  origin  from  the  internal  iliacs. 
They  pass  to  the  umbilicus  and  thence  to  the  placenta  by  the 
cord.  Blood  is  at  first  returned  from  the  placenta  by  two  um- 
bilicial  veins,  but  these  soon  fuse  into  one. 

Object  of  Placental  Circulation. — Since  the  activity  of  the 
respiratory  and  alimentary  tracts  has  not  been  established,  their 
functions  must  be  performed  by  those  of  the  mother  and  the 
necessary  materials  supplied  from  her  blood.  Consequently 
there  must  be  a  continual  passage  of  fetal  blood  to  and  from  the 
placenta  to  discharge  effete  matter  and  to  absorb  nutriment. 
Certain  modifications  of  the  circulatory  apparatus,  not  requisite 
after  birth,  are  necessary  to  bring  this  about. 

Course  of  Fetal. Circulation. — The  umbilical  vein  containing 
blood  enriched  with  oxygen  and  other  materials  enters  the  body 
at  the  umbilicus  and  passes  to  the  under  surface  of  the  liver. 
Here  it  divides  into  two  branches.  The  larger  joins  the  portal 
vein  and  enters  the  liver;  the  smaller  is  the  ductus  venosus,  which 
enters  the  ascending  vena  cava. 

The  ascending  vena  cava,  when  it  enters  the  right  auricle, 
therefore,  contains  blood  from  the  lower  extremities,  blood 
which  has  come  from  the  placenta  directly  through  the  ductus 
venosus,  and  blood  which  has  come  from  the  placenta  indirectly 
through  the  liver.  Considering  that  blood  from  the  body  of  the 
fetus  is  venous  and  that  blood  directly  from  the  placenta  is 
arterial,  the  contents  of  the  ascending  vena  cava  are  mixed  when 


PLACENTA L  CIRCULATION 


357 


they  enter  the  heart.     The  Eustachian  valve,  together  with  the 
direction  of  the  entering  current,  causes  the  blood  from  the 


FIG.  no. — Diagram  illustrating  the  circulation  through  the  heart  and  the 
principal  vessels  of  a  fetus.     (From  Yeo  after  Cleland.) 

a,  umbilical  vein;  b,  ductus  venosus;  /,  portal  vein;  e,  vessels  to  the  viscera;  d, 
hypogastric  arteries;  c,  ductus  arteriosus. 

ascending  vena  cava  to  pass  through  the  foramen  ovale  into  the 
left  auricle. 

Blood  from  the  upper  extremities  (impure)  enters  the  right 


358  REPRODUCTION 

auricle  through  the  descending  vena  cava.  The  Eustachian 
valve  and  the  direction  of  the  current  here  again  cause  this  blood 
to  enter  the  right  ventricle.  There  is  supposed  to  be  very  little 
mingling  of  blood  from  the  two  venae  cavae  as  it  passes  thus 
through  the  right  auricle.  At  the  same  time  the  blood  which 
has  entered  the  left  auricle  through  the  foramen  ovale,  augmented 
slightly  by  blood  from  the  ill-developed  pulmonary  veins,  passes 
into  the  left  ventricle.  The  ventricles  now  contract  simul- 
taneously. 

Blood  from  the  right  ventricle  (impure)  passes  in  small  part 
through  the  pulmonary  artery  to  the  lungs,  but  chiefly  through 
a  tube,  the  ductus  arteriosus,  into  the  descending  part  of  the 
aortic  arch. 

Blood  from  the  left  ventricle  (mixed)  enters  the  aorta  and  goes 
to  the  system  at  large. 

The  vessels  going  to  the  head  and  upper  extremities  are  given 
off  from  the  aortic  arch  before  it  is  joined  by  the  ductus  arteriosus. 
Since  the  ductus  arteriosus  contains  impure  blood,  the  supply 
going  to  the  upper  extremities  is  purer  than  that  going  to  the  lower. 

Of  the  blood  which  passes  down  the  aorta  a  part  leaves  by  the 
hypogastric  arteries,  to  go  again  to  the  placenta,  while  the  other 
part  is  distributed  to  the  trunk  and  lower  extremities. 

It  thus  appears  that  the  liver  is  the  only  organ  of  the  fetus 
which  receives  pure  blood,  and  that  the  head  and  upper  extrem- 
ities are  better  provided  for  in  this  respect  than  are  the  lower 
parts.  This  may  account  for  the  relatively  large  liver  of  the 
fetus,  and  for  the  fact  that  the  upper  extremities  are  better 
developed  than  the  lower. 

The  ductus  arteriosus,  ductus  venosus,  foramen  ovale,  Eusta- 
chian valve,  hypogastric  (umbilical)  arteries  and  the  umbilical 
vein  are  the  organs  which  distinguish  the  placental  circulation, 
and  they  all  partially  disappear  after  birth,  as  will  be  imme- 
diately seen. 

3.  Adult  Circulation. — The  circulation  as  it  exists  in  the 


%          THE    SKELETON  359 

adult  has  been  described.  It  is  only  necessary  to  see  what 
changes  mark  its  establishment. 

When  the  child  is  born  detachment  of  the  placenta,  or  ligation 
of  the  cord,  stops  the  placental  circulation.  The  first  noticeable 
effect  comes  from  the  consequent  deoxygenation  of  the  blood. 
The  respiratory  center  is  stimulated  and  the  child  gasps  to  fill 
the  hitherto  collapsed  lungs  with  air.  Owing  to  the  diminished 
resistance  in  the  expanded  lungs,  the  pulmonary  artery  begins 
to  carry  most  of  the  blood  from  the  right  ventricle,  and  the 
ductus  arteriosus  commences  to  atrophy.  Before  birth,  too,  the 
Eustachian  valve  becomes  less  distinct  and  the  foramen  ovale 
partly  closes.  At  labor  a  kind  of  valve  guards  the  opening  of  the 
foramen  ovale  and  allows  the  escape  possibly  of  a  little  blood 
from  the  right  into  the  left  auricle,  but  none  in  the  opposite 
direction.  It  commonly  closes  about  the  tenth  day  of  extra- 
uterine  life.  The  ductus  arteriosus  is  reduced  to  the  condition 
of  an  impervious  fibrous  cord  between  the  third  and  tenth  days 
after  birth. 

The  hypogastric  arteries,  umbilical  vein  and  ductus  venosus 
are  closed  between  the  second  and  fourth  days.  That  part  of 
each  hypogastric  artery  between  the  internal  iliac  and  the  upper 
lateral  part  of  the  bladder  remains  in  adult  life  as  the  superior 
vesical  artery;  the  part  between  this  point  and  the  umbilicus 
is  that  which  atrophies.  The  umbilical  vein  remains  as  the 
round  ligament  of  the  liver.  The  ductus  venosus  is  represented 
by  a  fibrous  cord  in  the  fissure  for  the  ductus  venosus  in  the  liver. 

The  Skeleton.— The  appearance  of  the  notochord  and  of  the 
protovertebrae,  or  somites,  has  been  observed.  The  notochord 
becomes  a  thin  line  of  soft  cartilage,  around  which  the  bodies 
of  the  vertebra  are  developed,  though  it  does  not  itself  become 
those  bodies.  The  protovertebrae  were  seen  to  lie  longitudinally 
on  either  side  of  the  notochord.  These  grow  around  the  neural 
canal  dorsally  and  the  notochord  ventrally  to  form  the  vertebra?. 


360  REPRODUCTION 

From  them  also  are  developed  the  muscles  and  skin  of  the 
back. 

The  cranium  is  developed  as  a  modification  of  the  vertebral 
column. 

All  the  bones  are  in  early  fetal  life  cartilaginous  or  membran- 
ous. Centers  of  ossification  appear  at  one  or  more  points  in 
each  bone. 

The  bones  of  the  extremities  are  not  at  first  separate.  They 
bud  out  from  the  upper  and  lower  parts  of  the  trunk,  to  be  sub- 
divided later. 

Nervous  System. — The  origin  of  the  nervous  system  has  been 
indicated  in  describing  the  neural  canal.  The  mesodermic 
cells  multiply  and  fill  the  tube,  until  only  the  canal  of  the  spinal 
cord  is  left.  Headward  the  neural  canal  terminates  in  a  dilated 
extremity,  which  soon  becomes  divided  into  three  vesicles,  an- 
terior, middle  and  posterior.  From  these  are  developed  the 
different  parts  of  brain.  Some  of  these  parts  develop  much 
more  rapidly  than  others,  and  we  thus  account  for  the  predomin- 
ant size  of  the  cerebrum.  At  first  there  are  no  cerebral  convolu- 
tions, but  later  the  cavity  of  the  cranium  seems  too  small  for  the 
brain  and  the  characteristic  infoldings  occur. 

The  eye  is  formed  by  the  projection  of  the  optic  vesicle  from 
the  side  of  the  anterior  brain  vesicle. 

The  internal  ear  is  formed  by  the  projection  of  the  auditory 
vesicle  from  the  posterior  brain  vesicle. 

The  alimentary  canal  is  formed  by  being  pinched  off  from 
the  mesodermic  layer  of  splanchnopleure.  It  communicates  for 
some  time  by  means  of  the  vitelline  duct  with  the  umbilical 
vesicle.  When  cut  off  from  the  latter  it  is  a  straight  tube,  occupy- 
ing the  long  axis  of  the  body  just  in  front  of  the  vertebral  column, 
and  is  divided  into  the  foregut,  hindgut  and  a  central  part. 
Later  it  communicates  above  with  the  pharynx  and  mouth  and 
opens  below  upon  the  external  body  surface  (anus).  The  liver 


FETAL   DEVELOPMENT  361 

and  pancreas  are  developed  from  prolusions  from  the  sides  of  the 
duodenum. 

The  bladder  has  been  seen  to  be  that  part  of  the  allantois 
which  is  constricted  off  and  remains  in  the  body. 

The  lungs  are  developed  from  the  esophagus  and  at  first  lie 
in  the  abdominal  cavity;  but  the  formation  of  the  diaphragm 
fixes  them  in  the  thorax. 

The  kidneys  are  developed  from  the  Wolman  bodies.  These 
bodies  are  embryonic  structures  only.  Each  is  a  tube  lying 
parallel  to  the  vertebral  column  on  either  side  of  it.  This  tube 
consists  of  a  collection  of  tubules,  which  unite  to  form  a  common 
excretory  duct.  This  duct  joins  the  corresponding  one  from  the 
opposite  side  to  empty  into  the  alimentary  canal  opposite  the 
allantoic  stalk.  Outside  the  Wolman  bodies  are  two  other 
ducts,  the  ducts  of  Miiller.  They  also  enter  the  intestine. 

The  Wolman  body  finally  gives  place  to  the  kidney,  from 
which  the  ureter  is  developed. 

In  the  female  the  ducts  of  Miiller  become  the  tube,  uterus  and 
vagina.  In  the  male  they  atrophy. 

Just  behind  the  Wolman  bodies  are  developed  the  ovaries 
or  the  testes,  as  the  case  may  be. 


The  development  of  a  few  of  the  organs  has  thus  been  simply 
referred  to. 

Satisfactory  explanation  of  these  procedures  can  be  given 
only  in  extended  works  on  embryology,  and  this  section  may  be 
closed  with  the  subjoined  table  of  development,  which  is  abbre- 
viated from  one  by  Heisler : 

First  Week. — Segmentation  and  passage  of  ovum  to  uterus. 

Second  Week. — Ovum  in  uterus.  Decidua  reflexa  present. 
Entoderm  and  ectoderm  layers  formed — also  mesoderm.  Em- 
bryonal area,  primitive  streak  in  primitive  groove.  Chorion 
and  villi.  Amnion  folds.  Umbilical  vesicle  partly  formed. 


362  REPRODUCTION 

Vascular  area.  Two  primitive  heart  tubes.  Gut  tract  partly 
formed. 

Third  Week. — Body  indicated.  Dorsal  outline  concave. 
Vitelline  duct.  Amnion.  Allantoic  stalk.  Visceral  arches. 
Heart  divides.  Vitelline  circulation  begins.  Gut  tract  still 
connected  with  umbilical  vesicle.  Liver  evagination  begins. 
Anal  plate.  Pulmonary  protrusion.  Wolffian  bodies.  Neural 
canal.  The  brain  vesicles.  Optic  and  otic  vesicles.  Olfactory 
plates.  Notochord. 

Fourth  Week. — Flexion  of  body.  Yolk-sac  largest  size. 
Somites  well  formed.  Allantois  grows.  Vitelline  circulation 
complete.  Allantoic  vessels  developing.  Pharynx,  esophagus, 
stomach  and  intestine  differentiated.  Pancreas  begins.  Pul- 
monary protrusion  bifurcates,  Ventral  roots  of  spinal  nerves. 
Limb  buds  apparent. 

Fifth  Week.— Umbilical  vesicle  begins  to  shrink.  Cord 
longer  and  spiral.  Length  of  fetus  two-fifths  of  an  inch. 
Primitive  aorta  divides  into  aorta  and  pulmonary  artery.  Intes- 
tine shows  loops.  Bronchi  divided.  Ducts  of  Miiller.  Epider- 
mis. Olfactory  lobe.  Eyes  move  forward.  Limb  buds  seg- 
ment. Digitation  indicated. 

Sixth  Week. — Umbilical  vesicle  shrunken.  Amnion  larger. 
Vitelline  circulation  supplanted  by  allantoic.  Teeth  indicated. 
Duodenum,  cecum.  Rectum.  Larynx.  Genital  folds  and 
ridges.  Dorsal  roots  of  spinal  nerves.  Eye-lids.  Lower  jaw 
and  clavicle  begin  to  ossify.  Vertebrae  and  ribs  cartilaginous. 
Fingers  separate. 

Seventh  Week. — Body  and  limbs  well  defined.  Heart  septa 
complete.  Transverse  and  descending  colon.  Nails  indicated. 
Cerebellum  indicated.  Muscles  recognizable.  Ossification  in 
cranium  and  vertebrae  begins. 

Eighth  Week. — Head  somewhat  elevated.  Parotid  gland. 
Gail  bladder.  Mullerian  ducts  unite.  Genital  groove.  Mam- 


FETAL   DEVELOPMENT  363 

mary  glands  begin.  Sympathetic  nerves.  Nose  discernible. 
Additional  centers  of  ossification. 

Ninth  Week. — Weight,  three-fourths  of  an  ounce.  Length, 
one  and  a  quarter  inches.  Pericardium.  Anal  canal.  External 
genitals  begin  to  indicate  sex.  Ovary  and  testis  distinguishable. 
Kidney  characteristic.  External  ear  indicated. 

Third  Month. — Weight,  four  ounces.  Length,  two  and 
three-quarter  inches.  Chorion  frondosum.  Placental  vessels. 
Tonsil.  Stomach  rotates.  Vermiform  appendix.  Liver  large. 
Epiglottis.  Ovaries  descend.  Testes  in  false  pelvis.  Hair 
and  nails.  Development  of  different  parts  of  brain.  Limbs 
have  definite  shape. 

Fourth  Month. — Weight,  seven  and  three-quarter  ounces. 
Length,  five  inches.  Head  one-fourth  of  entire  body.  Germs 
of  permanent  teeth.  Distinction  of  external  genitals  well 
marked.  Spinal  cord  ends  at  end  of  coccyx.  Eye-lids  and 
nostrils  closed. 

Sixth  Month. — Weight,  two  pounds.  Length,  twelve  inches. 
Amnion  at  maximum  size.  Trypsin  in  pancreatic  secretion. 
Air  vesicles.  Eye-lashes.  Lobule  of  ear  characteristic. 

Seventh  Month. — Weight,  three  pounds.  Length,  fourteen 
inches.  Meconium.  Ascending  colon.  Testes  at  internal 
rings.  Cerebral  convolutions  evident.  Differentiation  of  mus- 
cular tissue. 

Eighth  Month. — Weight,  four  to  five  pounds.  Length, 
sixteen  inches.  Body  more  plump.  Ascending  colon  larger. 
Testes  in  inguinal  canal.  Skin  brighter  color.  Nails  project 
beyond  finger  tips. 

Ninth  Month. — Weight,  six  to  seven  pounds.  Length, 
twenty  inches.  Meconium  dark  green.  Testes  in  scrotum. 
Labia  majora'in  contact.  Spinal  cord  ends  at  laSjtl  lumbar 
vertebra.  Ossification  centers  completed. 


r 
' 


INDEX. 


Abducens  nerve,  285 
Absorption,  from  stomach,  81 

from  intestines,  96 
Accommodation,  ocular,  314 
Adipose  tissue,  14 

formation  of,  178 

value  of,  179 
Adrenal  glands,  33 
Afferent  nerves,  233 
Air,  amount  necessary,  161 

alterations  of,  in  lungs,  150 

cells,  137 

composition  of,  148 

diffusion  of,  in  lungs,  148 

vesicles,  137 
Albuminoids,  175 
Allantois,  347 
Alveoli,  capacity  of,  147 
Ammonia    compounds    antecedents 

of  urea,  204 
Amnion,  the,  346 
Amniotic  cavity,  347 
Amylopsin,  102 
Anabolism,  172 
Animal  heat,  184 

loss  of,  by  evaporation,  189 

radiation  of,  189 

relation  of,  to  force,  185 

source  of,  185 

specific,  187 

total,  187 
Antehelix,  318 
Anterior  chamber  of  eye,  312 

elastic  lamella,  310 

fundamental  fasciculus,  241 

radicular  zone,  241 
Aphasia,  269 
Aqueous  humor,  313 
Arachnoid,  236 
Archenteron,  339 
Arteria  centralis  retime,  312 


Arterial  circulation,  47 

effect  of  respiration  on,  164 
Arteries,  47 

elasticity  of,  48 
Arytenoid  cartilages,  133,  324 
Asphyxia,  162 
Auditory  canal,  external,  317 

center,  323 
Auditory  nerves,  288 

terminations  of,  321 
Auerbach,  plexus  of,  97 
Auricle,  left,  44 

right,  44 
Axis  cylinders,  222 

Bacteria  in  digestion,  120 

Bartholin's  duct,  72 

Bellini,  straight  tubes  of,  197 

Bertin,  columns  of,  195 

Bile  ducts,  108 

Bile,  in  digestion,  105 

functions  of,  115 

properties  and  composition  of, 

no 

Bilirubin,  in 
Binocular  vision,  316 
Bladder,  207 

absorption  of,  207 

structure  of,  207 
Blastoderm,  339 
Blood,  the,  24,  35 

alterations  of,  in  lungs,  157 

amount  in  body,  35 

arterialization  of,  150 

coagulation  of,  40 

color  of,  35 

composition  of,  36 

functions  of,  35 

plasma,  36 

platelets,  39 

serum,  36 


365 


366 


INDEX 


Bone,  1 6 
Bone  marrow,  18 
Bowman's  capsule,  196 
Brain,  the,  251 

membranes  of,  236 
Breathing  (see  Respiration) 
Broca's  convolution,  270 
Bronchi,  136 

capacity  of,  148 
Bronchioles,  137 
Burdach,  columns  of,  247 

Capillaries,  the,  48 
Capillary,  importance,  50 
Carbohydrates,  6,  176 

final,  products  of,  176 
value  of,  in  nutrition,  176 
Carbon,  amount  in  excreta,  182 
dioxide,  amount  exhaled,  154 
amount  in  blood,  157 
condition  of,  in  blood,  155 
discharge,  151 
gain  of,  in  lungs,  150 
inhalation  of,  161 
interchange  of,  in  lungs,  156 
source  of,  exhaled,  154 
monoxide,  inhalation  of,  161 
Cardiac  cycle,  43 
length  of,  43 
Cartilage,  15 
hyaline,  15 
white  fibrous,  15 
yellow  elastic,  15 
Cauda  equina,  238 
Cecum,  117 
Celenteron,  339 
Celom,  343 
Cell,  i 

properties  of,  3 
structure  of,  2 
Centrifugal  nerves,  230 
Centripetal  nerves,  230 
Cerebellum,  the,  274 
anatomy  of,  274 
fibers  of,  274 
function  of,  274 
peduncles  of,  274 
Cerebral  localization,  268 
Cerebro-spinal  axis,  235 

system,  216 
Cerebrum,  the,  260 


Cerebrum,  cells  of,  263 

convolutions  of,  263 

fibers  of,  263 

fissures  of,  260 

functions  of,  270 

lobes  of,  260 

motor  centers  in,  268 
paths  from,  268 

sensory  centers  in,  262 
paths  to,  270 

special  centers  in,  269 
Cerumen,  32 
Cervical  ganglia,  298 
Cholesterin,  in 
Chorda  dorsalis,  343 

tympani,  288 
Chordal  folds,  343 

plates,  343 
Chorion,  350 

frondosum,  350 
Choroid  coat  of  eye,  310 
Chyme,  100 
Ciliary  muscle,  311 

processes,  311 
Circulation,  the,  41 

pulmonic,  41 

systemic,  41 

Circumvallate  papillae,  316 
Claustrum,  258 
Cochlea,  322 
Colloids,  123 
Colon,  117 
Colostrum,  31 
Common  bile  duct,  109 
Complemental  air,  147 
Conjunctiva,  310 
Connective  tissue,  1 1 
Convoluted  tubules,  197 
Cornea,  310 
Corona  radiata,  258 
Corpora,  quadrigemina,  2  59 

striata,  256 
Corpus  luteum,  331 
Corti,  rods  of,  322 
Coughing,  144 
Cranial  nerves,  275 
Creatin,  205 
Cricoid  cartilage,  133 
Crossed  pyramidal  tracts,  245 
Crura  cerebri,  256 
Crystalloids,  123 


INDEX 


36? 


Cutaneous  respiration,  160 

sensations,  center  for,  269 
Cutis  vera,  210 
Cystic  duct,  109 

Death,  172 

Decidua  menstrualis,  351 

of  pregnancy,  351 
Defecation,  121 
Deglutition,  78 

mechanism  of,  79 

nervous  control  of,  80 
Dendrites,  216,  222 
Descemet,  membrane  of,  310 
Descendens  hypoglossi,  295 
Diet,  amount  of,  182 

determination  of,  181 

necessary  constituents  of,  182 
Dietetics,  181 
Diffusion  in  lungs,  148 
Digestion,  68 

gastric,  81 

intestinal,  96 

object  of,  68 

processes  in,  70 
Direct  cerebellar  tract,  241 
Discus  proligerus,  330 
Dreams,  301 
Ductus  arteriosus,  358 

communis  choledochus,  109 

venosus,  359 
Dura  mater,  236 
Dyspnea,  161 

Ear,  the,  317 

drum,  318 

external,  317 

internal,  319 

middle,  318 
Ectoderm,  338 
Efferent  nerves,  280 
Eighth  nerve  (see  Auditory) 
Elasticity  of  arteries,  48 
Electrical  stimulation  of  nerves,  234 
Elementary  tissues,  7 

derivation  of,  7 

varieties  of,  8 

Eleventh  nerve  (see  Spinal  accessory) 
Embryonal  area,  340 
Encephalon  (see  Brain) 
Endocardium,  42 


Entoderm,  339 
Enzymes,  68 

characteristics  of,  69 

classification  of,  69 

mode  of  action  of,  70 
Epiblast,  338 
Epidermis,  209 
Epiglottis,  135 
Epinephrine,  34 
Epithelial  tissue,  7 

varieties  of,  8 

ciliated,  9 

columnar,  9 

glandular,  n 

modified,  9 

neuro,  n 

squamous,  8 

stratified,  8 
Esophagus,  78 
Eupnea,  161 
Eustachian  valve,  357 
Excretion,  197 
Expiration,  143 

causes  of,  143 

forced,  144 

effect  of,  in  blood  pressure,  165 
Expired  air,  composition  of,  151 
External  capsule,  258 
Eye-ball,  anatomy  of,  310 

movements  of,  308 

protection  of,  307 

Facial  nerve,  286 
Fats,  as  foods,  66 

end  products  of,  177 
Fauces,  78 

Feces,  composition  of,  120 
Fecundation,  336 
Ferrein,  pyramids  of,  195 
Fertilization,  336 
Fetal  membranes,  346 
Fibrin,  40 
Fibrous  tissue  13 
Fifth  nerve  (see  Trifacial) 
Filum  terminale,  238 
First  nerve  (see  Olfactory) 
Foods,  63 

classification  of,  64 

fate  of  in  body,  173 

how  absorbed,  128 

potential  energy  of,  185 


368 


INDEX 


Foods,  where  absorbed,  128 
Fourth  nerve  (see  Patheticus) 

ventricle,  252 
Fovea  cen trails,  312 
Fungiform  papillae,  316 
Funiculi  graciles,  252 

of  Rolando,  252 

Gall  bladder,  109 
Gases  in  intestine,  127 
Gastric  glands,  cells  of,  85 
nerve  supply  of,  89 
structure  of,  85 
varieties  of,  85 

juices,  action  of  on  foods,  92 
properties  and  composition  of, 

90 

secretion  of,  86 
Gastrula,  339 
Glands,  27 

adrenal,  33 

agminate,  100 

intestinal,  96 

gastric,  86 

mammary,  31 

of  Brunner,  99 

of  Lieberkuhn,  99 

parotid,  71 

salivary,  71 

sebaceous,  30 

secretion  in,  2  9 

solitary,  100 

sublingual,  71 

submaxillary,  71 

sweat,  210 

thyroid,  32 

Glisson's  capsule,  105 
Glomeruli,  renal,  195 
Glosso-pharyngeal  nerve,  289 
Glycocholic  acid,  in 
Glycogen,  formation  of  in  liver,  112 
Golgi,  corpuscles  of,  230 
Goll,  column  of,  247 
Gustatory  center,  269 
Graanan  follicles,  329 

Hairs,  210 
Haversian  canals,  17 

systems,  18 
Hawking,  145 
Hearing,  sense  of,  317 


Heart,  anatomy  of,  42 

beats  of,  43 

contractions  of,  43 

development  of,  355 

diastole  of,  43 

innervation  of,  46 

sounds  of,  46 

systole  of,  43 

valves  and  openings  of,  44 

work  of,  45 

Heat  of  body  (see  Animal  heat) 
Hemoglobin,  38 
Henle,  loops  of,  197 

sheath  of,  220 
Hepatic  artery,  105 
Hiccough,  145 
Hippuric  acid,  205 
Hunger,  seat  of,  64 
Hydrochloric  acid,  90 
Hydrogen,  inhalation  of,  161 
Hyperopia,  315 
Hyperpnea,  161 
Hypoblast,  338 
Hypoglossal  nerve,  294 
Hypoxanthin,  205 

Ileo-cecal  valve,  116 
Impregnation,  336 
Incus,  319' 
Infundibula,  137 
Innervation  of  vessels,  51 
Inspiration,  141 

causes  of,  142 

effects  of  on  blood  pressure,  164 

muscles  of,  143 

Inspired  air,  composition  of,  151 
Interlobular  veins,  106 
Internal  capsule,  259 

respiration,  158 
Intestinal  glands,  99 
Intestine,  digestion  in,  96 

divisions  of,  97 

movements  of,  121 

nerve  supply  of,  117 

structure  of,  119 
Intralobular  veins,  106 
Intrapulmonary  pressure,  140 
Intrathoracic  pressure,  146 
Iris,  the,  310 

Katabolism,  172 


INDEX 


369 


Kidney,  blood  supply  of,  199 

structure  of,  192 
Kinetic  energy,  186 
Krause,  end  bulbs  of,  228 

Labyrinth,  bony,  319 

membranous,  321 
Lacrymal  apparatus,  308 

duct,  308 

glands,  308 

sac,  308 

Lactates,  discharge  of,  205 
Lacteals,  98 
Large  intestine,  117 

digestive  changes  in,  119 

divisions  of,  117 

movements  of,  121 

structure  of,  119 
Larynx,  133,  324 

nerve  supply  of,  326 
Laughing,  145 
Lenticular  ganglion,  284 
Leucocytes  (see  White  corpuscles} 
Lieberkuhn,  crypts  of,  99 
Liquor  amnii,  347 

sanguinis  (see  Blood  plasma) 
Liver,  anatomy  of,  104 

histology  of,  108 

lymphatics  of,  no 

nerve  supply  of,  no 

vessels  of,  105 
Lumbar  ganglia,  298, 
Lungs,  138 

capacity  of,  147 
Lymphatic  glands,  59 
Lymph,  57 

course  of,  57 

flow  of,  6 1 

properties  and  composition  of, 

59 
Lymph  vessels,  origin  of,  57 

Macula  lutea,  312 
Malleus,  319 
Malpighian  bodies,  195 

pyramids,  195 
Mammary  glands,  31 
Mastication,  71 
Maturation,  32 
Meckel's  ganglion,  284 
Medulla  oblongata,  251 


Medulla  oblongata,  centers  in,  254 

functions  of,  253 

gray  matter  of,  253 

pyramids  of,  251 

relation  of  cord  tracts  to,  253 

white  fibers  of,  253 
Meissner,  corpuscles  of,  229 

plexus  of,  97 
Membrana  tympani,  319 
Menstruation,  334 
Mesoblast,  340 
Mesoderm,  340 
Metabolism,  171 

conditions  influencing,  179 
Micturition,  207 

center  for,  208 
Milk,  human,  32 
Mitral  valve,  45 
Mixed  lateral  column,  241 
Motor  oculi  communis,  278 

paths  from  cerebrum,  266 
Muscular  contractions,  23 

physiological  characteristics,  23 

tissues,  19 
Myopia,  315 

Nails,  the,  210 
Nasal  duct,  308 
Nerve  cells,  221 
centers,  221 
fibers,  217 

action  of  electricity  upon,  234 

afferent,  232 

classification  of,  230 

degeneration  of,  239 

directions  of  currents  in,  233 

efferent,  231 

individuality  of,  221 

medullated,  217 

non-medullated,  219 

properties  of,  230 

rates  of  conduction  in,  234 
terminals,  225 

between  epithelial  cells,  227 

in  bulbs  of  Krause,  228 

in  Golgi's  corpuscles,  230 

in  glands,  226 

in  hair-follicles,  226 

in  Meissner's  corpuscles,  229 

in  Pacinian  corpuscles,  227 

in  plain  muscle,  226 


370 


INDEX 


Nerve  terminals,  in  striped  muscle, 

225 
in  tactile  memsques,  229 

trunks,  219 
Nervous  system,  the,  214 

development  of,  360 

divisions  of,  216 

general  functions  of,  214 
Neural  canal,  342 
Neuroglia,  216 
Neurons,  216 

communication  between,  223 
Ninth  nerve  (see  Glosso-pharyngeal) 
Nitrogen,  amount  necessary,  182 
Nitrogenous  equilibrium,  174 
Nitrous  oxide,  inhalation  of,  161 
Nutrition,  171 

Olfactory  bulb,  276 

cells,  276 

center,  269 

nerve,  276 
Olivary  bodies,  251 
Omphalo-mesenteric  vessels,  355 
Ophthalmic  ganglion,  284 
Optic  center,  269 

commissure,  277 

nerve,  277 

thalami,  258 

functions  of,  259 

tracts,  277 
Osmosis,  123 
Otic  ganglion,  284 
Ova,  328 

Ovary,  secretion  of,  34 
Ovulation,  333 
Oxidation  in  the  body,  171 
Oxygen,  amount  consumed,  153 

amount  in  blood,  157 

condition  of  in  blood,  157 

entrance  of  into  tissues,  159 

loss  of  in  lungs,  148 

Pacini,  corpuscles  of,  227 
Pancreas,  anatomy  of,  100 

histology  of,  101 

internal  secretion  of,  103 

nerve  supply  of,  103 

secretion  in,  103 
Pancreatic  juice,  101 
Partial  pressure  of  gases,  156 


Patheticus  nerve,  280 
Pepsin,  91 
Peptones,  92 
Pericardium,  42 
Periosteum,  18 
Perspiration,  212 
Pharynx,  132 
Pia  mater,  236 
Pinna,  317 
Pituitary  body,  34 
Placenta,  352 
Placenta!  circulation,  356 
Pneumogastric  nerve,  290 

influence  of  on  respiration,  168 

stomach  and  intestines,  168 
Pons  Varolii,  255 

functions  of,  255 
Posterior  chamber  of  eye,  312 
Prehension,  70 
Presbyopia,  315 
Pronucleus,  female,  233 

male,  336 
Proteids  as  foods,  66 

circulating,  174 

final  products  of,  174 

tissue,  174 
Proteoses,  92 
Pro  to  vertebrae,  344 
Ptyalin,  74 
Pulse,  the,  53 
Pupil,  the,  311 

Ranvier,  nodes  of,  218 
Reaction  of  pupil,  315 
Receptaculum  chyli,  57 
Rectum,  118 
Red  corpuscles,  37 
Reflex  action,  248 
Refraction,  ocular,  313 
Reil,  island  of,  262 
Renal  tubules,  197 
Rennin,  91 
Reproduction,  327 
Reserve  air,  147 
Residual  air,  147 
Respiration,  131 

abnormal,  161 

afferent  nerves  of,  167 

center  for,  167 

costal,  146 

cutaneous,  160 


INDEX 


371 


Respiration,  diaphragmatic,  146 

effect  of,  on  blood  pressure,  163 

efferent  nerves  of,  169 

external,  132 

influence  of  vagus  on,  167 

internal,  131,  158 

mechanism  of,  138 

modified,  144 

nervous  control  of,  166 

object  of,  131 

organs  of,  132 

rate  of,  145 

rhythm  of,  144 

sounds  of,  145 

types  of,  146 
Restiform  bodies,  251 
Retina,  312 
Rolando,  fissures  of,  260 

Saliva,  functions  of,  71 

properties  and  composition  of, 

73 
Salivary  glands,  71 

histology  of,  72 

nerve  supply  of,  74 

secretion  in,  76 
Salts,  65 
Schwann,  sheath  of,  217 

white  substance  of,  218 
Sclerotic  coat  of  eyes,  310 
Sebaceous"  glands,  30 
Second  nerve  (see  Optic  nerve) 
Secretion,  27 

external,  29 

internal,  29 

paralytic,  76 
Segmentation,  337 

cavity,  337 

nucleus,  337 
Semicircular  canals,  322 
Semilunar  valves,  44 
Sensations,  common,  304 

special,  305 
Senses,  the,  304 
Serum-albumin,  36 

-globulin,  36 

Seventh  nerve  (see  Facial) 
Sighing,  145 
Sight,  sense  of,  307 
Sigmoid  flexure,  117 
Sixth  nerve  (see  Abdttcens) 


Skin,  excretion  by,  208 

functions  of,  208 

structure  of,  209 
Sleep,  300 

vascular  phenomena  of,  301 
Smegma,  30 
Sneezing,  144 
Snoring,  145 
Sobbing,  145 
Sodium  salts,  65 

functions  of,  65 
Solar  plexus,  298 
Somatopleure,  340 
Somites,  344 
Speech,  325 

center,  369 
Spermatozoa,  327 
Spheno-palatine  ganglion,  284 
Spinal  accessory  nerve,  293 

cord,  237 

columns  of,  240 
commissures  of,  238 
cross  section  of,  238 
degeneration  in,  240 
functions  of,  247 
gray  matter  in,  238 
motor  paths  in,  243 
sensory  paths  in,  245 
special  centers  in,  250 

nerves,  295 

Splanchnic  nerves,  298 
Splanchnopleure,  340 
Starvation,  effects  of,  180 
Steapsin,  102 
Stenson's  duct,  72 
Stercorin,  in 
Stomach,  the,  81 

histology  of,  83 

movements  of,  94 

nervous  supply  of,  96 
Straight  tubules  (renal),  197 
Striated  muscle,  19 

characteristics  of,  19 
Stapes,  the,  319 
Sublobular  veins,  107 
Submaxillary  ganglion,  284 
Succus  entericus,  116 
Supplemental  air,  147 
Suspensory  ligament,  313 
Sweat  glands,  216 
Sweat,  properties,  composition  of,  2 12 


372 


INDEX 


Sweat,  secretion  of,  212 
Sylvius,  aqueduct  of,  253 

fissures  of,  267 
Sympathetic  system,  216 
Syntonin,  92 

Tactile  sensibility,  305 

acuteness  of,  306 
Taste  beakers,  317 

sense  of,  316 
Taurocholic  acid,  in 
Temperature  impressions,  306 

of  body,  184 
Tenon,  capsule  of,  308 
Tenth  nerve  (see  Pneumo gastric) 
Testes,  secretion  of,  34 
Thermogenesis,  188 
Thermotaxis,  190 
Third  nerve  (see  Motor  oculi  com- 

munis} 

Thirst,  seat  of,  64 
Thoracic  duct,  57 

ganglia,  298 
Thorax,  138 
Thyroid  cartilage,  133 

gland,  32 
Tidal  air,  147 
Touch,  sense  of,  305 
Trachea,  135 
Tragus,  318 
Trie  us  pi  d  valve,  44 
Trifacial  nerve,  280 
Trigeminal  nerve,  280 
Trypsin,  102 
Turck,  column  of,  245 
Twelfth  nerve  (see  Hypoglossal) 
Tympanum,  318 

Umbilical  cord,  354 

vesicle,  346 
Urea,  202 

daily  discharge  of,  203 

formation  of,  203 
Ureters,  206 
Uric  acid,  204 

daily  discharge  of,  204 


Urine,  constituents  of,  202 

discharge  of,  206 

salts  of,  206 

properties  of,  202 

secretion  of,  199 

variations  in  amount  of,  206 

in  composition  of,  206 
Uriniferous  tubules,  197 

secretory*  changes  in,  202 

Vaginal  plexus,  105 
Vagus  nerve  (see  Pneumo  gastric) 
ValvulEe  conniventes,  97 
Vaso-motor  nerves,  51 

centers  for,  51 
Vater,  corpuscles  of,  227 
Veins,  48 

valves  of,  50 
Ventilation,  160 
Ventricle,  left,  44 

right,  44 

Vermiform  appendix,  118 
Vestibule  of  ear,  319 
Villi,  98 
Vitelline,  circulation,  355 

duct,  344 

Vitreous  humor,  3-13 
Vocal  cords,  133,  324 

sounds,  varieties  of,  325 
Voice,  production  of,  324 

Water,  65 

elimination  of  by  kidney,  201 
Wharton's  duct,  72 
White  corpuscles,  39 
Wirsung,  duct  of,  100 
Wolffian  bodies,  361 
Wrisberg,  nerve  of,  280 

Xanthin,  discharge  of,  205 
Yawning,  145 
Zymogen,  101 


